A few that I think would be good candidates:
A pair of Yasha shrimp gobies and an associated pistol shrimp. The shrimp can be purchased separately. I know from experience that tiger pistol shrimp work with these gobies. While these are not that common, the price is pretty reasonable. The gobies are out and about more than some other very pricey tiny gobies, and they are brilliantly colored.
They are not that rare or expensive, Hawaiian leaf fish are very unusual and not that common. You could might get by with a pair in a tank that size (though I would recommend a bigger tank). Really neat looking fish. You will probably have to feed live food - guppies and ghost shrimp. I've kept these several years and have never had one accept frozen.
A 12 gallon tank is really small. There are a lot of interesting rare and unusual fish I could suggest for 20 or 29 gallon tanks. But with a 12, you are really limited in space and how much bioload you can handle. There aren't even really that many COMMON saltwater fish you can put in a tank that size.
Also not rare, but really cool, are some of the smaller mantis shrimp, such as N wennerae. Try
www.stomatopod.com.
For all others, I have had the best luck as to quality, selection and service from liveaquaria.com. Diver's Den is the WYSIWYG section, and has the rarer fish, but try their Collector's Corner, and their regular menu of selections.
You might be able to select some really interesting inverts and corals, with just a few common fish (e.g., clown gobies, neon gobies).