the problem with using sand is it gets in the markers.
the best alternative i have seen is crushed gravel, on top of concrete.
The thing is, you have to wash out an indoor site quite regularly, and you need awesome ventelation.
There's this place that we go in Toronto called area 51 that is a great indoor field. The do crushed gravel and it works pretty good. They are built into an old airplane hanger, and the main door is netted off so when the weather is good they open the main door (which lets in the natural light) and let the air circulate.
Crushed Gravel isn't going to be cheap, you might as well invest in something like astro turf, our even out door carpet. If you wash it regularly and have ventelation to let it dry out properly it should last for a few years, of course patching any place with wear.
stuff like this would work:
indoor/ outdoor carpet