You can get the factory kit directly from Seadoo. On their website is a link to the Seadoo store somewhere. You can get these same kits from your local seadoo dealer, although they may have to order them. Also, you could get them for slightly less money on Ebay, or even online sites like Parker Yamaha.
Just make sure the factory kit is compatible with your ski. Once you do the install, you have to take it to a dealer so they can program your ski's computer to use the cruise control.