We use VetVits EquiFlex - have done for about 2 years. Really good, 1 tub lasts about 6weeks, (14.2hh pony), and costs £15.99. You get mailorder - google VetVits and I think you'll get their website.
You can get Corta Flex from www.netequestrian.com for a fraction of the price. My last tub of 2.5lb cost only £30 including the postage instead of over £55. It's called Corta Flx on the tub but is exacty the same thing.
You can also get if at a similar price on e-bay from "equimins" or something like that. They are also in America but it still only costs around £30 including the postage.
You should try Pernamax from www.maxavita.com. It really has worked for Bill. He's off his bute and the stiff swollen joints which he's had for months are amazing. He seems really happy again.