Try LiveAquaria if you want a large number... You can get 15 for $66 shipped, about $4.50 each including shipping.
Also, you might have luck using the scientific name... Ask your pet store if they use 'sunpet' as their distributer, and if they do, go to
www.sunpet.com and search it.
If you're looking (for example) for a dwarf spotted puffer, you won't find it. Look for
Carinotetraodon travancoricus (it's scientific name) and you'll find "Golden BB Puffer". Different common name, but the same fish.
Amano shrimp is trickier, but look for "Caridina Japonica" or "Caridina multidentata" which are both "Amano Shrimp" or "Alagae Eating Shrimp" depending on who you ask. Again, they're all the same species of shrimp.