My local Tschibo store have disc shaped (13inch diameter) wobble cushions this week for £2 for those needing them for SIT/OT. The plastic nobbles are not as softly rubbery as the expensive ones. They also have some (rather soft) foam seat wedges (£4). I can recommend their bed pads (cheap and wash and dry well - although they do not have them in at present. I have no idea how countrywide they are.
Thought I'd let you know.