I can't help you out with a bike myself, but segundamano and milanuncios are good starting points. There are loads of bike shops in spain though, and if all else fails then decathlon have reasonable bikes at decent prices - they're the best quality/price ratio for new bikes at the low to mid end. Retro bikes aren't as big here as they are in the uk, but they do exist. I'm nog aware of any retro-specific forums, but the largest bike forum in spain is foromtb, there is some retro stuff but mainly road.
FWIW, shipping a bike from the uk to here can be done for under 50 GBP, so if you find something you want in the uk and the seller is willing to ship abroad...
Let me know if I can be of any help
Cheers Foz I think I'm going to go down the retro route from the U.K. defiantly not new from over here. I've never bought a bike with any form of suspension never mind disks!
Thanks again ,Daz