There are multiple services to lease SteemPower. @Blocktrades (blocktrades.us) and @minnowbooster (minnowbooster.net) are the two biggest players. Blocktrades is cheaper at times, but minnowbooster is peer to peer so it might be cheaper at times. Theres also @master-delegator, a smaller program to lease SteemPower. This one is done differently using a bitshares UIA token which can be cashed in for a delegation.

You need to check the prices of all three places to make sure that you are getting the best deal, but they are constantly changing.

Source: I run @master-delegator so I know about the delegation business.