They seem to hold their value well so it may be worth getting a new one. Your local 'Freeads' type paper may be worth trying or this site has 2nd hand ones but are quite expensive: http://www.fieldandcountryantiques.co.uk/
This site: http://www.hunting-directory.co.uk/ has a forum where people sometimes have secondhand coats for sale.
Good luck.
I looked on eBay for ages, ended up getting a new one, did this by finding the make I wanted and doing a web search and calling all the dealers, got some good advise on the sizes as well.
HOYS is just around the corner if your going there are a couple of trade stands that may have one (there were last year), may even get a discount if your lucky.
Make sure you order one in plenty of time to give it time to arrive, I didnt and on the morning of the our local meet it had not arrived and i was waiting outside our local saddlery waiting for it to open!! Luckily he arrived early and let me in and i had to buy a very expensive one! I think only my wedding dress has cost more! :crazy: Dont tell my oh though :grin:
Thanks everyone, ican't afford a new coat, if daughter had stopped growing it would be a good investment. I'll keep looking but she'll have to use her £25 ebay jacket for now, perhaps i'll get her a decent waistcoat instead.