You will probably hate me but I've just found a Harris Tweed coat in a charity shop. I'm not posting to brag - I had drawn a blank everywhere else but had a hunch I might get lucky in a small market town so we took a day trip and there it was.
Also (sorry if you know this already) those towns usually have at least one country clothing store and some of them, I have discovered, are pure treasure caves. In one of them I tried on a men's tweed overcoat in XS. It was a bit big on the shoulders but otherwise fitted (I'm normally size 8-10) and I was seriously thinking of buying and getting it altered. It cost £250 - a bit over budget but I thought it was a very good price for 100% wool. I'm sorry I can't remember the brand but it was Irish - possibly Dubarry or Jack Murphy.
I had also been also tracking the Uniqlo JW Anderson range in case one of the tweed coats got returned and I've seen a couple come up but they're being snapped up quickly.
Toast also have a lovely one but it's about £350 - but will come down in the sales if any left.
I love a bit of tweed and I hope you find something.