When I was setting up my 75 for dwarfs, due to budget/shipping/history I decided to only get what I could source within an hours drive. I was keeping an eye out for keyholes, apistos, checkerboard, small growing geos....and ended up going for rams and kribs. Kribs killed the rams once they paired up. So. . .be careful and/or open to failure about how you go about this.
Others will be better about knowing about fish that only show up for mail-order and then rarely.
But, I got some orange spot sunfish, and I don;t know anyone who sells those, just had to catch 'em myself.