More than any other typo, yeah vs. yea errors make me want to light myself on fire. A very good friend and collaborator of mine (hi, Mark) is a great prose writer, but it took me the better part of a decade to get him to stop making this common error. There's a big difference between yeah and yea:
Yeah: Sort of sounds like "yah". This is the modern, informal way of saying "yes".
Yea: Rhymes with "nay". A very old-fashioned way of saying "yes".
Also, I see no problem using "yah" in place of "yeah".
Both yeah and yea are real words, so most spellchecks will not catch this error. If you're someone who makes this error, I recommend adding a step to your editing process where you Ctrl+F yea and scroll through your MS to make any needed corrections. I recommend against using Find + Replace to bulk convert yea to yeah, because the likelihood of creating more typos doing this is high (i.e. if you're not careful, "See you next year" becomes "See you next yeahr").