I just bought one from Maplin for convenience when ours failed. More cost effective sources are available, I'm sure.

IIRC, it was a FortecStar twin output job and I'd forgotten I'd changed it a couple of years back, so I guess it's working OK.
It really depends what type of dish you have and what mounting the LNB comes with.
Sky minidishes have a square lug that pushes into the end of the support arm. Most generic dishes have a circular clamp for the LNB. You can get adaptors to fit one to the other, but..
In theory, Sky minidishes require a different feed horn arrangement in the LNB because they are not circular but oval. In practice this probably doesn't make a huge difference and you may well get away with a generic LNB on an adaptor. I did.
When setting up, bear in mind that the LNB needs to be positioned accurately at the focal point of the dish and it needs to be skewed at an angle to give the best separation of the H and V polarisation signals. The amount of skew depends on your location.
It's a good idea to get a strength meter and "tune for maximum smoke" for dish azimuth and elevation, LNB distance from the dish, then swap between channels with H and V polarisation and try and get the best compromise of signal strength and quality by adjusting the skew. Repeat that sequence a couple of times to get the best signal that you can (even if it's properly aligned, you may well knock it out of alignment when wrestling with the LNB).
Kevin