Mothercare? Boots? Saw some plain white sleepsuits and vests in our Boots today (don't know if they sell them online though - their online range is v. limited - but you might find them in your local Boots - ours is a small one and I've never seen clothes there before - it was a new display of `Mini basics' - all plain white).
H&M are good for cheap T-shirts for boys but don't know if they do online.