Hi,
I have been over many times for the TT and Manx and always camped but recently I have done the Tour of Mann Rally in September and accommodation has been available!
For value and good service, try the Caledonian. Its on the Douglas prom so you can walk down into town for food and they serve a great breakfast. I think any of the hotels along the front will be pleased to accommodate you so it may be worth calling Steampacket holidays (as opposed to Steampacket ferries - same company but different department) who could possibly advise.
Anyway, here is the link for the Caledonian, I stayed last September and it was cheap and pretty good.
http://www.caledonia.im/Have fun, you cant go far wrong on the Island.