where to buy bathingsuit in tokyo

Does anyone know stores where I can buy relatively cheap bathing suits in Tokyo? Thanks in advance.

I'm staying at the imperial and didn't bring my bathing suit with me and could really go for a swim! I was going to hit up Ginza now and try to find one but some tips before would be awesome!

The sports departments of the major department stores would be a sure bet, but they might not be so cheap. Ginza is the last place to go for cheap clothing.
If you are after a cheap swimming costume, I would try the Kinkado department store in Ikebukuro. I'm not 100% sure that they sell swimming costumes, but prices are much lower than the big-name stores.

If it's not too far away from you try Lalaport.They have a few sports stores (Nike store etc) which sell nice, not too expensive ones.They also have a whole store of swimwear, with bikinis,swimsuits and beachwear, but it's a bit more pricey.