Well it really depends. I presume you have the 18-55 kit lens right now? If so, set it to 50mm and see if that will give you the magnification you need.
If you think 50mm is enough for you, do you need autofocus? If so, the obvious choice is the Pentax-F 50mm f/1.7 which sells on this forum for about $150. I'm no expert on Pentax zooms but I doubt you could get a fast autofocus zoom lens for this price which has a decent focal length for indoor sports (approx 50-135mm).
Of course, if you don't mind a bit of degredation of image quality, you could just set your ISO to 1600 and set your aperture on your kit lens wide open. I don't think the results would be horrible!
Edit: by 'fast' zoom, I'm talking f/2.8, not f/1.7!