I looked into this earlier in the year with PENSCO and they were not allowing futures trading through their self-directed IRAs at that time... their focus is marketing SD IRAs for investing in property and businesses. When I pressed their compliance officer for an alternative I was directed to Millennium Trust. As it happens I never moved forward with it, but hopefully this will give you a head start.
Yes, txuk is right, Millennium Trust is a good IRA company to go with. Last time I checked, they allow you to trade with all but $500. There's also Lincoln Trust, Entrust, and North Star Trust to name a few other alternatives.