If I put a 17T sprocket on my GS, what size chain would I need? 
(I saw mention somewhere of someone having an 18T; where would you find one of those?  Bikebandit only has 17s.)

Standard 110 link chain will work with 17/39T sprockets.

http://gstwins.com/gsboard/index.php?topic=44849.msg502495#msg502495

Maybe 18T front will too, check it with calculator:

http://tsubaki.com.au/tsscs/chain_length.htm

Rule of thumb is that you can add or remove 3 teeth cumulatively front and back compared to stock, and still use the stock chain. The other general rule is that it's best not to subtract more than 1 tooth from the front sprocket, as doing so wears the chain guide and puts more stress on the chain.

Keep in mind, that's the combined total front and rear. -1 +2 counts as 1 tooth total. So, pretty extreme gearing adjustments are possible with the stock chain; E.g. -1 +4, or -2 +5 (not recommended, also... Wheelie!)