I find that my children's feet tend to grow over the summed. I buy new wellies and welly socks in the autumn. Once the weather is warm again, we don't need the socks anymore, and the wellies fit for longer.
This is what the lady in Jo Jo maman bebe recommended. Buy the next size up to wear with welly socks over the winter. Then come spring when weather warming up and feet grown take socks out and they will still fit hopefully
I was planning to buy DS Hunter Wellies (he has had them for a couple of years now) and they tend to be a lot cheaper online - so wasn't planning to try them on (just order them). Will have a think. I suppose if I buy 10s and they are too big, I could put them away for a while.
qtpie if you're buying hunters get the correct size as they come up really big. All my dcs have them and I find they come up a size bigger. And yes to the wellie sock thing my dd2 is a 10 and her 9s still fit her with the socks in.