Hi looking for some advice or guidance. Where is the best place to find a rear shock for a 2009 model. Also roughly what's the cheapest price I could likely buy for? 

Would other make/model shocks fit? If so which ones? 

Any help would be appreciated. 

eBay is your friend. I suppose you could measure the shock to get the dimentions and then just buy one of the same size, but if you're looking for a second hand one from another bike then eBay if your best bet. Unless you want to buy a legit one from Yamaha, in that case just go to a Yam dealer and consult them.

Depending on the price it might be easier to have your shock rebuilt. Have a look at this listing:

http://www.ebay.co.uk/itm/YAMAHA-XJ600-DIVERSION-JYARJ1-2009-2010-Hagon-Adjustable-Mono-Shock-/301881466862?hash=item4649899fee:g:yAEAAOSwKtVWzxTk  (May not be the exact shock, but it will be very similar)

Those shocks are quite expensive, you may be able to take the one you have to a mechanics and have it repaired for less.

Yeah as Variomatic say's it's going to be expensive whatever way you decide to go I'm afraid, just think how lucky you are not to have an FJR AE, their shocks START at £5 grand!!!

You may be better trying the xj600 owners club for info