There's no real concept of a 'weaner' in sheep; they sell as stores (not yet ready, need feeding and growing time) or fat (ready to kill.) That's how it is in the livestock marts up here, at any rate. People just know which breeds and types will finish in one season and which will run on to Christmas or beyond.
If you aren't confident to pick ones which will finish by November yourself, ask the auctioneer to select a batch for you, or to give you an indication, or ask the drovers to, of which pens would suit you. They'll advise you on what is a reasonable price for the sort of thing you are looking for as well.
Assuming you are in Devon, then where you are, there'll be Texel cross lambs and Mules which I'd have thought ought to finish by November, providing that the Texel crosses aren't out of Hebrideans or something. And even then, unless they're late born, they may well finish by Nov on good grass.
As to price, you will get a shock I expect. People frequently pay more for wee store lambs than finished lambs are fetching in the ring next door, which seems bizarre - but when you see the prices the finished stores will fetch come Christmas time/New Year, it makes sense.
There will be a market for store lambs in the back end, as many dairy farms will run sheep on their ground to clean it over winter when the cattle are indoors, so you could also buy stores now and sell them again later - but I'm not sure you'd make much money out of that. Again, your auctioneer would advise.