If you want normal colours, I highly recommend just dyeing blanks. I'm not even all that crafty, but we did that for DS and ended up with some great clothes. Some tie-dye, some solids, and it really didn't take that much time. We got blanks from Dharma and dye from a local source as we didn't have a US address at the time. If you're in the US, you can get everything from Dharma.
We've always been very lucky with hand-me-downs, too, and we have an amazing second hand children's place just up the street from us, but for the occasional new item, MEC (cheaper if you are in Canada) has good plain stuff, for example, here is a plain long-sleeved onesie in various colours. In the US (and still available in Canada, but requires duty) LL Bean has similar stuff.