I just got a set of Sidas PU soft foam liners for my SPK's. There a good liner for the spk as they allow for some expansion, which is usally required to help fill the shell. Most of the other foam liners tend to be to narrow to fill the liner up, that is what I have found any way. I got the liners for $225 so not too a bad price. They make the boots fit so well, once you get a pair you will not ski in regular liners again, and last a good while. I usally found I would wear a shell out before the liner, so if you buy the same shell say the next year, you can just swap the liners out. You will need to buy a footbed preferable a decent custom one, as the foam liners come without.
Hope that helps mate