Try TKMaxx or Matalan They have nice leather picture frames should be some multi ones or maybe try ebay?
Just sponge off with a damp rag.Don't put anything except water on rag.It may work. Otherwise there is a dry powder cleaner for leather. Failing that, check with a Saddlers to see if they sell a leather soap you could use.
This depends on what type of leather you have. DIY is not recommended for this type of problem although it is possible to have it professionally done. DIY kits off the internet are very rarely successful and can be very damaging so should be avoided....