Daiso is a dollar-store-style retailer who sells extremely cheap goods under their own brands. You can get sushi-making supplies, toys, stationery, media storage, and beautiful Japanese bowls for, like, a buck each. If you happen to be near
one of the locations
(there are 9 in the U.S.; we visited the one in Seattle) it's definitely worth a quick browse. Especially now that the company has started selling DS accessories!
We can't really predict what will be available in which store, but there's no reason to believe that these Daiso-branded silicone DS covers and screen covers won't be available in the majority of their locations. We're a little uncertain about the screen covers, but at 315 yen ($3) it's not much of a gamble. As far as the DS covers go, a silicone cover is a silicone cover, and Daiso's is less than $2.