When needed, I used whatever I had in the fridge. Different yeasts have different attenuation and flavors produced. If you're going to leave your fermentors sitting out at room temp, you'll want to go with an ale yeast.
Safale 04 or 05 would probably be a good one to start out on.
http://www.fermentis.com/brewing/homebrewing/product-range/
Those are pretty common in local brew shops. Our liquor stores that carry some brewing supplies usually have one of both of those, so it should be easy enough to find.