To start, this setup cost me less than $30 dollars to do with free barrels. You can do it for cheaper with one barrel or using other parts. I designed this to be able to be expanded (10 - or maybe more barrels :-D) and to include a pump.
This took me a week to do because I could not decide / find all the right parts but once i got one adding the other took me about an hour to do. I'd suspect that this could be done in a matter of an afternoon with all the parts at hand.
Step 1: Your type of Barrel
There are a few types of barrels:
Plastic food barrels (usually pickles or olives come in them)
They have a screw on top and are usually black in color. (see picture 1)
These are also the safest if you are even thinking of using the water to drink out of. The rest should not or use caution actually drinking the water from them. I'll also talk about what I've found with types of roofs that you should be careful of.
Plastic cleaning barrels
They usually contain cleaning products for businesses since they buy them in bulk. Some may or may not wash out after cleaning them out. They usually have a 2" screw off cap or tapable cap and a 2 1/4" screw off cap. (this is the type i will be showing, they can be white, blue or sometimes another color)
Plastic industrial barrels
Usually are a few varieties, but all contain chemicals that are probably not safe for your garden or lawn and especially not safe for drinking. (They look like the barrels I'm using but are can be blue or another color)
Can have a variety of different goodies in them. They do not make good rain barrels because they easily will rust with water.
Make sure to check with the person you are getting them from if you are unsure of what the contents were.
You're probably also wondering where can I find a barrel or two?!? Now that's the fun part, since every city is different there are some possibilities where you can get them. Usually you can get them for free or relativity nothing.
If you have a food factory near by try them first, they usually have the pickle barrels or some sort of food barrel you could use
The next few places you could try are some smaller industry places that use a lot of cleaner. i got mine from a smaller place that uses cleaner to clean out coloring that is safe on the environment.
Craigs list in some places you can find people selling them from 10 - 100 dollars.
Now if your really getting disparate and you don't care about how much it costs you can do a Google search for them and find them for 75 - 180
If you don't want to have to do the work of creating one you can buy a ready made one also.
If I'm inaccurate in my statements please let me know. I'd like to provide correct information to everyone since it was harder to find for me.