I feel your pain; it seems like every store (okay, the 2 that are mom-and-pop types and the 2 big chain pet stores) around here carry on the average every day fish that everyone else has like neons and mollies and guppies and only a few cool and unusual fish. Then again, once in a while they have something really unusual.
However, I've found that just asking if they can do special orders will get me what I want. If I remember correctly, you are specifically looking for figure 8 puffers? Have you asked your local stores if they can special order them for you. It will save mucho money on shipping for you. Just confirm what they will be asking for price, though. I once asked about diamond tetras and congo tetras (making my decision on which to order based on price). I was able to get the diamonds for only $3.69 each whereas they congos were going to be over $20 each! So, yeah, confirm the price with them and then make sure it matches what they quoted you (once the diamonds came in they wanted $3.70 something but gave me the price quoted when I asked about it).
Like phoenix asked, are you dead set on brackish? If you can go with freshwater, you can open yourself up to some more options, and if you can't afford to get a larger tank, then you might also be finding yourself not being able to afford the salt mix, hydrometer (or refractometer), special foods as most of these guys will only take frozen or live foods, etc. If you are ready and able to do brackish, then phoenix already listed several.
