For nicotine base, on a tight budget, this thread has some info:
http://www.e-cigarette-forum.com/forum/diy-e-liquid/601513-nicotine-deals.html
The last page or two of this thread might also be helpful:
http://www.e-cigarette-forum.com/forum/diy-e-liquid/298547-nicotine-comparisons.html
Not all nicotine is equal, and some people strongly prefer the taste of certain brands over others, but what is best for you will depend on your preferences and budget, so some reading may help.
For VG and/or PG, many people here recommend Essential Depot for large quantities. A search of this subforum for that will provide links and info. Regarding flavorings, yeah, probably 2/3 of the threads in this subforum deal with that, but if you are just starting, you can likely get a few flavorings from the same place you get the nicotine. Same for the VG and PG, if you just want a little to try your hand at mixing, you can get smallish amounts of that at the same time as your nicotine.
Maybe others here can provide some specific recommendations regarding flavoring sources. I have become rather ambivalent about flavorings, since the longer I vape, the more I like just plain old nicotine, VG, and a bit of water (and my budget likes it too).