I have 13 New-In-Box Giant branded inner tubes for 24" rims. They are the Downhill/wide tubes for Downhill tires/bikes. The size is 24 x 2.4/2.75 and they are 2.25mm thick and have the standard (Schrader) inflation valve. These are new and unused, although the boxes aren't in the best shape from storage.
These aren't as big as a typical inner tube that one might use in a pool or lake but are thicker than a typical bicycle tire and are similar to motorcycle size. So, they are big enough and thick enough for use in pools or lazily floating down the river or lake (which everyone seems to be doing since the COVID outbreak started).
The price is $10/ea or all for 13 for $100. Feel free to ask questions and if you would like to come see 1 and blow it up to get an idea of what it looks like, you are more than welcome.