Couple of eBay options, both around £30-£40:

http://cgi.ebay.co.uk/DUFF-NORTON-24V-4 ... _710wt_932

http://cgi.ebay.co.uk/Linak-linear-actu ... _782wt_932

The second one looks like it's been modified with a windscreen wiper motor and looks a bit bulky, but if it's putting out 600kg at 65mm a second, that's pretty good.