there is a 240 tire for sale on the board a few posts down... I got one on ebay once for $165.00 shipped.... good luck..
I have two used 180 tires that you could like glue together and make a 360 out of!! No really I will be on the outlook for ya for a good deal on a tire!
http://cgi.ebay.com/ebaymotors/AM42-AVON-VENOM-250-40VR-18-250-40-18-REAR-TIRE-4959812_W0QQcmdZViewItemQQcategoryZ35578QQihZ004QQitemZ140156240284QQrdZ1QQsspagenameZWDVW