IOM's can be very expensive new, anywhere from £500 up to £1800 just for a hull, Even 2nd hand ones can hold there value so be wary.
You can try ebay you may get lucky,
but i would suggest putting an advert up here ...
https://myaukforsale.wordpress.com/iom-class/with your budget, you will normally get contacted, as those boys like to swap out to new hulls regularly, our club uses it and we get a lot of good hits within the £2-300 price range with full rigs.
Last one was £180 with all 3 sail sets, and only required a TX set to get working, overall the boat was in excellent condition, only a few scrape marks.
Or you could try to make one yourself using the Traplet wood kit..
http://gb.trapletshop.com/nimbus-mk3-setNylets do the Full rigs (sails, MAst, Boom, Jib etc)
http://www.nylet.co.uk/catalog.pdfOr if your after something a little cheaper and easier to mange (smaller boat) maybe try a wee nip.
http://gb.trapletshop.com/wee-nip-setAgain wood pack from Traplet and Sail can be brought pre-made from Nylets, they are so simple to build, sail and maintain only being a single sail. i promise!, don't believe me this is mine...
(full build blog of it here..
http://www.thercmodelboatforum.com/t887-wee-nip-build )