Here is how to build a steel stand to hold your whiteboard. It has wheels on the bottom so that you can push it around and it is stable so that you can write on it without it wobbling around. Here I built three stands, each whiteboard is 4ft high and they are, respectively, 6ft, 5ft, and 4ft wide. The stands are positioned about 2ft off the floor so that they are perfect for comfortable writing.

The materials needed to build one whiteboard stand are:

Slotted steel angle brackets. two 6ft, six 3ft (or cut three 6ft in half), and two that can be bolted together to run the length of the whiteboard. For example, for my 5ft whiteboard I used two 3ft angle brackets and bolted them together in the middle to run a total of 5 ft.
One threaded rod the length of the whiteboard. For example, for my 5ft whiteboard I used a 6ft rod and cut it to 5 ft.
A piece of slotted flat metal.
Four wheels. Use the ones that can turn without the bolt heads interfering. Imagine a bolt head in each of the holes on the wheel bracket and make sure the wheel still turns freely. Best is to put a bolt in the hole and turn it to make sure before you purchase the wheels. You may want to get two of the wheels to be locking ones (although I don't think that locking wheels are very useful unless the board is much lighter than the ones I build here. More often than not the locking wheels are just a pain in the ass.)
A bag of 3/4 inch long 7/16 hex bolts (get the size that fits in the slots of the steel angle brackets).
A bag of nuts for the above hex bolts.
A bag of washers for the above hex bolts.
6 1.5 inch hex bolts of the same gauge as the 3/4 inch ones above.
A step drill bit for metal.
Rubber washers and nylon specialty washers (for the front of the whiteboard.
Angle grinder for cutting the metal.
Wrenches.
Hammer.

The project will probably take about 3 hours.

Step 1: Base Brackets

Take the two 6ft angle brackets and attach a 3ft bracket to the bottom of each one as shown in the picture. Use a flat piece of slotted metal and a 90 deg. angle. You will end up with two T shapes.

Step 2: Angle Braces

This step takes a bit of time. You have to lay out the 3 ft pieces as shown and then mark where you want to cut them so that they can attach both at the bottom and the center shaft. Notice that one side of the center shaft (the one that attaches behind) requires a cut down the center of the angle of the angle bracket.

Step 3: Clean Up the Cuts

Now that you have attached the angle braces so that you have two triangle looking things, you need to take a hammer and pound in the pointed and sharp pieces that are sticking out.

Step 4: Drill the Board

Now turn the whiteboard so that it is face down on a table and then lay the uprights along each side and mark the holes where you want to attach it. Note that it looks better if the inside of the angle bracket points to the center of the board. I usually have the bottom hole going through the aluminum of the brush holder, then another half way up, and the last at the top.

Once the holes are marked you drill them with the step drill to the gauge of your bolts.

Then flip the board over and drill the other side to the same gauge as the bolts (a step drill will leave a cone shaped hole so you need to drill the same hole from both sides to get a uniform gauge all the way through)

Step 5: Attach the Uprights

Now bolt the board to the brackets. Use 1.5 inch bolts.

Use the rubber washer and the nylon washers on the white part of the board as shown.

Don't over-tighten the bolts with the rubber washers. Just tighten until turning the nut will also turn the bolt head through your fingers and then hold the bolt head and give it one more turn.

Step 6: Threaded Crossbar

Now attach a threaded crossbar across the bottom connecting the two brackets just above the center. This will stabilize it against bending inward or outward as you move the board. Use the angle grinder to cut any excess crossbar.

It is best to use a tape measure and measure the exact distance between crossbars across the back of the whiteboard and then make sure the threaded crossbar is that same length -- it won't likely be the correct length naturally since the base is quite heavy.

Step 7: Attach the Wheels

On the back side of the brack base, attach a piece of angle bracket connecting the two sides. This is the one where I used two 3ft angle brackets and bolted them in the middle to the correct length. I also had to use the angle grinder to cut the ends so that the bolt holes lined up.

Then attach two of the wheels as shown. I only bolted 3 of the wheel bolt holes. Also I had to use the step drill to increase the size of the bolt holes in the wheels so that my bolts would fit.

The wheels on the front side of the whiteboard are attached using only two of the wheel bracket holes and only two bolts. Use diagonal holes so that the center of the wheel is under the bracket.

There is no crossbar across the front side of the base -- that is where someone will be standing.

Step 8: Finshed!

You can now stand up the whiteboard and it should be movable and fairly stable. Of course you must still be careful when you are moving it. Above I show the smallest one I made today: the 4x4 one. The front page of this instructable shows the 4x5 and the 4x6 ones.

Note that these brackets can cost from $600 to $1200 for the size of whiteboards that I used here. I think that under $100 and 3 hours of your time is probably well worth it. (Making 3 whiteboard stands simultaneously can be done in about 5 hours)