I have gotten rainbow rock at walmart and at Hancocks (or Michaels, I get them confused). But you should be able to call local craft stores and ask if they have Rainbow Rock procion dyes.
I've had great success with them. I'm not a professional, so I can't magically make things look professional-- but I have fun and my kids have made their own shirts while I'm also dying the diapers.
And yes, ubcpfs dye very well.