Hi Golf Machine, 

You could try 3balls.com. They normally have good deals. I have only heard good things about this site. 100% legit site. The reviews are really good. I have actually purchased from them twice. I recently bought a Titleist 910 D2 for $97 and a Titleist 910 FD for $59. They listed the clubs both in very good condition and when they came into the mail they came better than I expected. I believe they have a condition scale of new, fine, very good, and good. A lot of the clubs are extras from golf shops, PGA Professionals, golf retailers and custom fitters where they get stuck with more clubs then they need at the end of the golf season. 

Golf Mart or any of the Roger Dunn Golf Shops.   Probably used, though.  I saw a clearance on SM4 wedges as the SM5s come in.

Another site to check out is GlobalGolf.com.  They sell used clubs of all brands, including Titleist, and what is available changes over time.  I picked up a used SC Fastback 1 from them and was quite pleased with the condition, the cost, and the speed of shipping.

Golf Machine -

If your local fitter is an authorized Titleist fitter, he can check with Titleist to see if there are any remaining 910F heads available (the cry of desperation...). 

Besides 3Balls, if you are careful, you can check e-Bay and look for internet resellers that guarantee authenticity and have a large number of transactions.  Individuals are a little more risky.  You may find 3Balls options for bidding at a lower price than the main website.  I've used 3-4 sites so far without an issue, although not always for a used Titleist club.

Right now, there are huge selections of new, 3 release old clubs from the guys releasing multiple times a year.  Titleist 910s will used.  Happy hunting, and if the more common combination shaft/head aren't offered, wait a week.