I found a large selection of solid color tees at Lands End. Their kids' catalog had a lot. At the beginning of summer, I ordered red, white, bright pink, orange and bright green.
They have a more limited color selection available now, but here it is:
Also, I don't know how "nice" you want them to be, but the Hanna Andersson short sleeve boxy tees are really high quality and fit for a long time. http://www.hannaandersson.com/style....7C26%7C2%7C%7C
They are probably twice the price of the Lands End shirts. They're also certified by Oeko Tek, so there's that benefit in terms of pesticides/toxic substances. My daughter has some of the boxy tees and some of the Lands End Super-Ts and we use both. The HA boxy tees are much nicer in comparison, but the Lands End are very nice and I think comfy, too.
Hanna Andersson also carries something (for girls) called "Power of a Great Tee" that is a t-shirt with a slightly fitted shape and a less boxy sleeve cut. They're not tight, but more feminine. They were on sale recently, but I think the sale is over. They also wear exceptionally well and for a long time--my daughter has some short- and long-sleeved ones from last year (in her old size) that still fit her comfortably this year.