This Instructable deals largely with 100 series Land Cameras. This includes all cameras with a model number between 100 and 455. Countless quantities of these cameras were manufactured and sold between 1963 and 1976.
You can still find them at thrift stores, garage sales, estate sales, antique stores, and online (think Ebay
Your best bet is to find one at a garage or estate sale. Although antique cameras sometimes tend to fetch a little bit of money, most people think you can't buy Polaroid film anymore and will offload these cameras on the cheap.
The current market for Land Cameras is good because no one wants to buy them and everyone wants to be rid of them. This makes your job of acquiring one all the easier.
I got this whole camera kit for $5, and with the warning, "You know, you can't get film for those anymore."