Platinum Traders (www.platinumtrading.net ) supposedly gives one up to 10x buying power. I have no experience with them, so I couldn't give you any feedback on them. One big catch that I do know of though, is that you don't really get your own account with them. You only have a subaccount, which is part of one big account. The problem with this is that your money is not insured like it is in a normal account. Also, they are not actually broker/dealers, they are just a group of traders with pooled money that works with an actual broker.
10:1 is the minimum provided by most prop-retail firms (like Genesis, Bright, Assent etc). You can often get much more than that with some experience and trust.