There are a lot of options. You can get kits from a variety of sources. First, though, what kind of guitar do you want to make? Acoustic or electric. You can get kits for either one from
Stew-Mac or
Luthiers Mercantile, though the electric kits from LMI will take a lot more work, and I would not recommend them if you have not already got some experience. You can also get parts from other sources. For an electric, I would highly recommend parts from USA Custom Guitars (I have one of their necks on the Strat I recently built myself - I could have made my own, but given the cost of my time it was cheaper and faster to buy it from them). Warmoth is another high quality source. Cheaper, but also lower quality, parts can be had from Mighty Mite, WD Music products, and Allparts. Frankly, they are not that much less expensive, and their quality is significantly lower. If you buy from USA Custom Guitars or Warmoth, I HIGHLY recommend that you have them finish the parts for you. They do an excellent job, their rates can not be beat, and a good quality finish is one of the hardest things to do, particularly for a first timer. A "Parts" guitar, as they are frequently called, is a very good learning experience, and can help you get some of the most important skills needed. It is not, however, inexpensive. Then again, there isn't really a way to make a good quality DIY guitar which is inexpensive.
The first thing to do, however, is read up on it. The internet is a poor source for information on this stuff because there is a very high amount of bad information. I would suggest a couple of books. There are a number of good ones out there. If you are thinking an electric,
Electric Guitar Construction and
Make Your Own Electric Guitar are good. If you are thinking acoustic,
Guitarmaking is good (though their neck making method is awfully fussy, and you would be well advised to ignore their distaste for rasps, files, and sandpaper), and either of Irving Sloan's books
Classical Guitar Construction and
Steel String Guitar Construction are classics of the trade (though Sloan's stuff may be a bit outdated. Art Overholtzer's book is also a classic, though also a bit outdated, fairly controversial, and as far as I know out-of-print.
If you have any specific questions, feel free to ask here, and either Muttley or myself (both of us are professional guitar builders) will be happy to answer your questions. (At least I know I will, and I assume Muttley will feel the same, as he always has).
Most of all, though, I strongly encourage you to give it a try. It is one of the most rewarding things you will ever do (at least, if you like guitars), and a lot of fun. Just remember two things - you will NOT save money over buying a comparable quality guitar (no kidding here, it will not happen, so don't even think about doing it to save money), and there is a high chance that you will not get everything perfect. Don't think of it as a way to get a great guitar cheap, think of it instead as a fun learning experience. If you think about it that way, you will not be disappointed if things don't end up perfectly, and if things DO come out perfectly, you will be that much happier.
I think that is probably enough for now.
Light
"Cowards can never be moral."
M.K. Gandhi