My LFS keeps them in stock all the time... $0.99 ea regular price. The sunday paper has buy-one get-one-free coupons there for all fish under $5.00. So I end up getting them for like $0.50 ea.
Bumblebees need to be kept with small salt-tolerant fish that won't bother them. I think that knight gobies are right out as they get too big. Bumblebees rarely get more than like an inch or so, and so many other species look at them as lunch.
Many popular livebearers are salt tolerant and will do well with BBG's. Mollies, swordtails and platies come to mind. The sailfin mollies though tend to get pretty big so I might suggest skipping those. While they are a cichlid, I've had good luck cohabitating kribensis with BBG's.
Figure 8 puffers and GSP's are, IMHO, completely inappropriate tankmates for BBG's.
The best plants for a BW tank are plastic ones.
The specific gravity for BBG's isn't critical, as long as it is saltier than FW and fresher than SW. 1.005 to 1.015 is a perfectly acceptable range. I try to stay around 1.008 to 1.010.
Live and/or frozen food is a must. While some will tolerate flake, they really don't do well at all unless provided a variety of quality frozen and live foods. Mine have taken a particular liking to brine shrimp and glassworms (aka mosquito larvae). Some of the smaller BBG's have a hard time with brine shrimp due to the size of it so try to size your food to fit your fish. Frozen baby brine is probably more appropriate for BBG's than adult brine shrimp. Bonus if you can score (and even maintain a culture of) daphnia.