@mumcreates
I have copied below what I said to OP in my PM, however I realise with amazement that it is nearly 10 years ago that we were in this position, so current Eton mums will have more up to date, and more useful, info than me - it was just that I still have 2 W&J suits which we haven't got around to returning yet.
This is my contribution, for what it's worth!
The thing about Eton is that it is a very well-oiled machine that you get sucked into. They don't really do uniform sales like prep schools (I don't think they did anyway, you might have needed to be more local than we were) they have their own system.
What happens is that you will be summoned to a housekeeping meeting with your dame in May, and she will pack you off to the house's preferred tailor (they say something like 'I am not allowed to recommend a specific tailor, but...' and then you go to the tailor your dame hasn't recommended). Your son will be measured up and then everything is ready and waiting for him, all name labels sewn in (the dame will allocate a special laundry number, so make sure you find out this number if you are having your own done or your son will never get any of his stuff back) and will be in his room on his first day. It's very impressive!
The tailors usually recommend that you buy one new and one second hand suit, and they do all the upkeep and dry cleaning during his time there so the suits last pretty well unless they are outgrown. The suits were actually not as expensive as we had been expecting, it's all the sports stuff, ottomans and other stuff that adds up
Of course none of this stops you from being prepared and picking up some bargains beforehand and many boys wear hand me downs from older brothers so it's not a problem.