for just cheap, any plain gauge spokes in 1.8mm (15ga) or 2.0mm (14ga) from wheelsmith, DT swiss or sapim will serve it's purpose.
If you want to build custom, you might as well buy some good, double butted spokes to build with. The straight gauge you can buy from shops and online aren't worth it, when you consider the price of double butted and their longevity.
If you want just want to learn how to build, buy some ready-made wheels from places like velomine, then disassemble and reassemble them. Much cheaper than trying to learn how to build with parts bought separately.
It's not that I want to discourage you from doing what you want, but I just want to throw out some options, in case you haven't considered them.