A lot depends on how many step dancers and the nature of their program. Talk to them first, they might have their own.If you are strapped for cash, ask the local Festival Organisers, Clog teams etc if they would loan one for a donation. If it's for a demo, for two or three performers, an 8 x 4 sheet of heavy ply/chipboard might suffice. The dancers have to be happy with the area and the surface.

Stu