Are there any game guides or walkthroughs available for Disciples? I've tried googling but I can't seem to find anything.

There seem to be some in gamefaqs. It's probably the best place to finding walkthru's/faqs etc.

For Disciples 2, the ones on GamesFaqs by Mister Sinister are quite good.
No idea for D1...

Gamefaqs has a FAQ for 1 but it is written by a guy who didn't really understand some of the game's minor details or was writing it from memory. Also dropped it before finishing his walk through so expect no real help there.
Some tips for me on 1: Your armor stat is godly, maxing your level + buffs = near unstoppable so focus on just one or two parties, don't underestimate the power of spells, ignore enemy capitols as they are not worth the trouble of taking out (even if they are an instant mission win) a races support unit (those upgraded from buildings on the swerely tab) help define each races role so don't ignore them, your thief can steal items and spells from venders and lastly never forget to transfer leaders and items