Apart from the 2 I mentioned, Stirling castle's impressive and, if you're a fan of Monty Python and the Holy Grail, you may recognise various parts of Doune Castle. Culzean Castle (pronounced Cullane) is more of a stately home if you prefer that sort of thing. There are loads more but I'm no expert so someone else may be able to recommend their favourite.

As to Castle Forbes, I've never seen any of their stuff for sale so I suspect you'd have to get it from the net or go there directly - it's in Aberdeenshire, somewhere I've never really been, except in transit so I don't know if there's much else to see there.