It seems to me that you have a couple of ways of going. There are several internet companies that have been in business for a long time, who specialize in WM points.
www.redseason.com has a great reputation (who I bought from recently), as well as another website, who's name I can't remember (I'm sure someone else will chime in with who it is shortly). These folks might not be the cheapest, but they have great reputations and you should have a painless, quick transfer.
How do you feel about ebay? There's always someone who is advertising memberships there. I know that PA (fullup) has memberships for sale on ebay every so often and has a great reputation (as well as being an active participant here). This would probably get you your best price.
It will be harder to find 5000 points - Trendwest sells 6,000 points as a minimum membership. There are some 5000 point memberships out there, but it is more common to see 6,000.
Do make sure you read the fine print - WM charges $150 for a transfer fee, and there isn't a need to go through a title company, unless you want the security. Check to see if whoever you are buying from is tacking on additional fees, and what they are for - it ultimately affects the price.
Good luck with it!