Penguina - if you wish to buy one, then there are prices on the Tr*an$@1r site (but think in terms of around £7-800 a shot + maintenance, which is significant). Much better to rent one (either from your club, from someone like another aviation supply company, the name of which I can't remember
or from a local survival equipment emporium (if you have one!) ).
I would not recommend getting one from the 'guy down the pub' 'coz sure as eggs are eggs, when you need it -
you need it and then is not the time to find it's leaking, the CO2 bottle's naffed or the baler's missing. As Whirly pointed out, a raft is not the easiest thing to get in and out of - consider having a course in a swimming pool where you try getting into one without capsizing!