Where can you get 48/96GB 3090s? I know there are 24GB customized RTX 2080s (there were a few on ebay even last I checked) but I've never even heard of the 3090 being upgraded before.

eBay or aliexpress. A handful of people have bought it on r/locallama. You can search for 4090 48GB (I misremembered, they’re modded 4090, not 3090). There’s 96GB 5090s