For flock you can actually buy sawdust and then buy the smallest paint pot you can find for the colour; rubbishinrubbishout on youtube has a tutorial on this.
As for
GS I buy from back-2-baseix BUT they're an Aussie company so shipping may make it unfeasible for you
http://stores.ebay.com.au/Back-to-Base-ix?_rdc=1 100g of greenstuff for $16 (36 inches) is about the best price you'll find for Aus but not necessarily for the USA.
There's many US sites out there anyway, for Au we have very few (
PF,
CC etc) so pin vice doesn't matter where, and bases I just look on ebay as the sky's the limit for how cheap you can get them (may even be worthwhile in some cases to buy poorly painted models and using the bases as you get free bitz too).
As for ebay sellers I think hoard o bits, aa-wargames and spikey bits are usually good but I'd love to hear of any better ones.