Best bet is to order directly from the USGS. You can find the maps, a directory, and ordering info on their website. Takes 1-2 weeks to get them, but you can get maps for anywhere in the USA. Since I have collected many topos over the last 30 years, this is the method I use to get those strange ones I don't have - I can't justify the cost of software, a good printer, and good paper.
There are still a few outdoor retailers who stock USGS topo maps; check on the USGS website to see if there are any in your area. Only limitiation on this is the retailers usually only sell the more popular maps. There used to be one in Orange County near Angel Stadium, but it went out of business in 2007.
Larger REI stores have electronic map kiosks where you can print a custom map (on waterproof paper) of an area you'll be visiting. The kiosks use National Geographic's TOPO! software. Cost is about $10/map; not a bad deal if you're an infrequent purchaser of new maps.