Apple do now supply replacement key-cap sets as service spares to their authorised repair centres so although purely sales Apple stores may not be a able to, any authorised Apple service provider (AASP) SHOULD be able to provide replacement key caps to any unibody MacBook Pro. I believe the 2009-2012 pre-retina key caps are only supplied by Apple as full US format sets so they can't supply country specific keys like the UK 3 key. The newer clip types on the Retina MacBook Pros are supplied as international sets covering a number of European country formats.
Having said all that, this does not mean they actually will supply them as this is entirely up to them and they may want to charge you a fee for fitting it.
To complicate things further, before the introduction of their own ‘butterfly’ design, Apple sourced their keyboards primarily from two different companies; Darfon and Sunrex, who used different clip designs under the keycap. Apple used keyboards from both manufacturers on the same range of Macs and the keys are not interchangeable, so just because you get a key from the same type of MacBook Pro you have, it does not mean it will fit. Apple supply the service spare recap sets mentioned above, to AASPs in these two manufacturer versions with two different part numbers. They call them type D or S to match the manufacturer name.
If you get no joy from your local AASP, every type of keycap from every model of Mac (including the older models that Apple don't supply service spares for) can be sourced from The Bookyard.
In the interests of full disclosure, I do work for The Bookyard but as this is the most comprehensive Apple keycap store on the internet, I assume referring to it is justified by it's relevance.