White Gas is mainly normal gasoline which has not been formulated for using in automobile engines. It is more filtered and generally purer than what you get out of a gasoline pump. It is called by several names in camping stores such as Coleman Fuel, Crown Camping Fuel, and MSR Fuel. It can be sold in 1 gallon cans and in 1 quart bottles or cans. The big stores such as Wal-mart will have the gallon can and sometimes the quart bottle. Camping stores often have the quart bottles or will sell you a quart out of an open gallon can at the store. Often, you can buy a quart off of someone at the campgrounds that has a gallon can with them, they are the people with the big RV rigs.
+1 on availability of the quart cans. I tend to find them more at independent camping stores and at REI and EMS. Only once was I low and forced to buy a gallon. I left behind at a campground what I could not carry for others to use. And I also had a camp store offer to fill my fuel bottle from a gallon can. If you don't want to carry two fuel bottles, you can simply carry the can with the remaining fuel in your pannier and top off your bottle each time you use the stove until the can is empty.
Another handy use for white gas is starting campfires.