I built a sub some years ago, very simply with no maths. It worked reasonably I guess. I haven't attempted anything since although i've been tempted. I always thought it would be difficult to get a professional or even acceptable finish, you'll need some good carpentry skills unless you intend to cover them in carpet or something (but presumably not with stereo speakers). I know a couple of suppliers, I doubt they are competitors so here you go:
https://secure.wilmslow-audio.co.uk/ (UK based)
http://www.partsexpress.com/ (USA, but bigger selection)
You should choose drivers you think are suitable, then select the best enclosure type for them (would love to try a transmission line myself, but they are the most complex). Use Thiele small parameters etc to calculate enclosure sises, port location; frequency response of the driver to tune a ported enclosure. Search google for tutorials unless you are already familiar. It is possible to achieve great results for moderate cost, good luck.
https://secure.wilmslow-audio.co.uk/ (UK based)
http://www.partsexpress.com/ (USA, but bigger selection)
You should choose drivers you think are suitable, then select the best enclosure type for them (would love to try a transmission line myself, but they are the most complex). Use Thiele small parameters etc to calculate enclosure sises, port location; frequency response of the driver to tune a ported enclosure. Search google for tutorials unless you are already familiar. It is possible to achieve great results for moderate cost, good luck.