After the states ran out, then the US territories began...Puerto Rico, Guam, US Virgin Islands, Washington DC (I might be forgetting others). Anyway, the US Mint didn't make as many of these as the state quarters due to a glut in the market, so to speak, after ramping up production of the 50 state quarters. In the few years they've been out, I've only seen a handful (And save every one I can).
Likewise, the mint then started the "America the Beautiful" series, highlighting national parks in each state. Again, production is cut back from the numbers of 50 state quarters released, so there aren't a lot out there.