I can't believe Chelsea would be interested in Walker, but I'd happily ship him out for the figures quoted (£15M).
It's difficult, top class full-backs are rare, so it's probably a case of (even more than normal) finding a player who fits the system and can just do his job. For me, that's not Walker, particularly with Bale likely ahead of him. We just need someone who can defend properly and get the ball out to Bale, quickly and precisely.
I know this is slightly hollow without suggestions, but I only know the Premier League, and can't think of anyone suitable from there, apart from perhaps Maicon. I'd need to see more of him to form a proper opinion, but he looked interesting (at least pre-City) and he's available, albeit a relatively short term solution.