Replacement MTB rear derailleur I need to replace a Shimano Deore RD-M531 rear derailleur which has a long cage (9 speed cassette). I can replace the derailleur no problem, but my knowledge isn't great and am just not sure what to replace it with. This is an old bike and I would be happy with second hand to minimise costs. I would buy the same but none available on line. Any suggestions?
Thanks
Matt
Shimano Deore RD-M591 is a comparable part which you can still find -- this is probably what I'd use. If you want to go a bit cheaper, Shimano Acera M360 will work too. 

You can use any 7-9 speed Shimano rear derailleur to replace another 7-9 speed Shimano rear derailleur provided it has the appropriate capacity (*) for your setup (at least big chainring - small chainring + big cassette cog - small cassette cog) and largest cog rating, since the cable pull is the same. 10 speed onwards has a few more cases. 

(*) Capacities tend to be conservative -- you can usually exceed them by a bit if you don't use absurd gear combinations.