According to
Digital Choices, you can get one for £20 if you've already got the kit from:
FREESAT from Sky
Don't know about your other question. If it's a Freeview PVR, I'd have thought not as the only signals you're going to get from Sky are analogue (UHF) antenna and composite signals and I'd guess that Freeview PVRs can only record digital channels. But I don't have one and have never looked at them so I could well be wrong.
[Edit] Or you could replace the Sky box with a Freesat PVR box.