Sock Dreams is pretty popular. They ship from Portland, Oregon and shipping is free in the US. Their prices are reasonable. They stock a large variety of colors, including some pastel. I own their "O Basics" in white and pink and the quality is alright. They are very thick but do not stay up at all without sock adhesive in the form of tape or glue, or garters. I also own their "Pink Knees" and they are HUGE. The area where your foot is supposed to be is around 11 inches long. Personally, I just cut off some of the toe and sewed the hole up, making them the right length. They are now comfortable and stay up better than the O Basics, but still not too well.
Joy Of Socks sells very reasonably-priced OTKs in black and white and some other colors like wine and navy. They don't sell pastel colors, however, you could buy white ones and dye them.
Absolute Socks sells some solid colors, stripes, argyle, and some printed socks (non-lolita specific, skulls and such). The prices are alright, more expensive than Sock Dreams or Joy of Socks though. No pastels as far as I can tell. Also, their categorizing is horrible, but you can use the search bar to find OTKs.
Artisan Socks has a few pairs of OTKs. Odd colors, odd patterns, and expensive, but there are some pairs that could be easily coordinated.
Socks 4 Life has some solids and stripes. Mostly darker colors- black, brown, wine, navy.
Tabbi Socks sells a good variety of OTKs in both solids and prints. (They also sell the popular skeleton-print socks!) They sell some socks in pastels like pink, mint, and what looks like sax (can't be sure with the photos!) Link provided by tanzy13
If you have anything to add- either details on any of these places, or other offbrand stores that sell OTKs, please comment! I will add to this list and credit the original poster. :) As you can see, I've been unable to find much in terms of offbrand OTKs in pastel colors so please let me know if you know of places that have them.