Try the specialists rather than ebay. 9ib would be ok ish. depending if you want it to hold its shape completely when sat on it.
with 12lb foam, clench your fist and push down hard, 12 lb will compress 2 inches ish where as 9lb will compress 4-5 ish,
top choice would be:
01543 275051 Email
kate@quayslideleisure.com or louis if he still there
at Quayside and Leisure Ltd
OR
Transcal Ltd
Firth Road
Houstoun Industrial Estate
Livingston and Bristol
EH54 5DJ
T +44 (0)1506 440 111
F +44 (0)1506 442 333