It’s been going on for a while. Search YouTube or the web for 48gb 4090 (this is one of the most popular modded Nvidia cards), Nvidia of course never officially made a 4090 with this much memory.

There are some on sale via eBay right now. The memory controllers on some Nvidia gpus support well beyond the 16-24gb they shipped with as standard, and enterprising folks in China desolder the original memory chips and fit higher capacity ones.

Go at ebay and search for RTX 4090 48GBs. There's plenty of them with prices around $3.5k