Well, it depends on what I am using them for.
If I need really sturdy, nice studs, I will order them from studsandspikes. Or even the ones you can buy from Hobby Lobby for like $1.50 (not for very many though) usually do quite nicely.
Otherwise, I look for clearance studded belts from places like Hot Topic, Rue 21 (where I got my last 5 belts for only $1 a piece!), etc. Then I take some jewelry pliers, or whatever tools I have (I shall post pictures at a later date because they are in my room at school and I am currently home) and I DE-STUD those belts and keep all the studs in a nice little zip lock baggy. Along with the rest of my craft/DIY supplies.