Totsy has some plain white swim diapers on sale for $4.75. The brand is "i. Play". (I have no idea if its a good brand but at less than 5 bucks you can't really complain, right?)
Clicky (not sure if it will link directly because it's an invite only site)
ETA: There are excellent reviews on these swim diapers on Amazon.
Re: PSA: Swim Diapers on Totsy
Crap, I just bought 2 of these at BBB for $9.99 each. Oh well. At least they have good reviews, that's good to know.