FYI we do provide parts for the external Apple USB and bluetooth keyboards including keycaps, scissor clips, silicone domes, battery caps and internal circuit boards. For keycaps, see here For other keyboard parts, see here. Thanks
For UK and the rest of Europe, the Bookyard provides new and used service spares for all Apple products searchable by the product serial number as well as the machine ID, family number or part number. The whole parts store is organised by Apple service range and mirrors Apple's GSX parts database.
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...
This does sound like a logic board fault but it is actually very difficult to confirm other than by elimination. It can be normal for the fan to momentarily spin when the power is connected to the machine and before the machine it turned on. It could simply be that the signal from the power button is not reaching the logic board. Every logic board has power-on solder pads that can be used to manually power not he machine by shorting them. You would have to Google where these are on your specific model, but this can rule out a faulty power button. Disconnecting anything non-essential helps eliminate other components like the Airport/bluetooth board & battery. Beyond that, the only way to be sure is either to replace the logic board or get your logic board fitted to an identical model MacBook Air that works but this is obviously not practical for most people.