A needy Englishman wrote:
Can't see any on eBay/Etsy. Where do they go? I want to buy them.
Generally, schools replace their singlets every few years. When they do, the school usually has a policy that the old ones get thrown away. This usually means the current kids on the team take the old singlets home with them as momentos. They usually keep them, because they're meaningful to them.
I think it would be pretty rare that they wouldn't end up taking all of the old ones and any others would be left over to be sold on ebay, etc.
Your best bet is to find individuals who don't want them anymore and are willing to sell/trade you. Maybe you'll get some emails.