Auctions-
Bacchus - already mentioned - the best source imo.
J Straker Chadwick, a one-time forum favourite but online only since Covid. Best to go to Abergavenny and collect in person if you win some rather that sort out delivery.
Bid for Wine - individuals list their own bottles via the platform. some bargains, some go high.
There are also several specialist retailers for mature wines. PM if you want details.