You might be able to do something with a boat trim tab actuator, if they were fast enough (I don't know how fast they operate, or whether you could speed it up easily (perhaps 24v instead of 12 ). http://cgi.ebay.com/ebaymotors/Brand...#ht_500wt_1374
A linear stepper would be cheaper than a linear servo. Sorry, no prices but you could ask these guys: http://www.h2wtech.com/stepmottor.htm
For linear servos, the I-force ones from Parker Automation are nice: https://buy.compumotor.com/e_search....nless%20Motors - they do have prices, and you can order online if you're feeling wealthy. The Parker Gemini drives are recommended for those. You'll need a linear encoder.
It would be worth figuring out what version you'd need and keeping an eye on ebay too - sometimes there's cheap, new linear motors with no drives. Grab a DRO scale encoder and something like the Granite Devices VSD-E drive to run it and you're set (that makes it sound so simple...
Ps - I should say, I do have an affiliation with Granite Devices. There are other drives that will run them too, but I don't know of any in the same price/performance bracket, which is why I suggested them.