Why buy them yourself? Just pay the people who already have them to find hashes for you. Pay noticeably better than the existing pools and they would line up at your door.

Miners are motivated by profit. They won't necessarily even know that you're trying to do a 51%, just that you're buying hashrate and are willing to pay them more than they're getting elsewhere.

edit: In fact, you could run a legitimate mining pool at a small loss for a while to build trust. Participating in your loss leader mining pool would be attractive to miners since the other pools, that skim some profit for themselves to support their operations, can't compete on price with one willing to lose a little bit of money. If you want, break it out into 3 "different" pools that you control so that it even looks to the naive like there's no 51% control. Then spring your trap.

51%ing Bitcoin is well within the resources of a motivated hedge fund, major corporation, or country. The motivation just isn't there right now.