There are a few listed on evil bay now. Your local bike shop should also be able to order them for you as well. I'd check with them. Google search, "24x3 bicycle tires", may also extend your search beyond ebay, too.
It seems to me that the 24x3 size has never been widely popular, and there are still several different options on the market. I'm not sure whether or not you can fit a 24x3.45" tire, but if you can, please consider the hoggy-g http://ratrodbikes.storenvy.com/col...-tires/products/883054-hoggy-g-tire-24-x-3-45 , which is available on the RRB store.
They are had to find in Australia. I'm not far off needing some and the local Electra dealer looked at me like I was sniffing glue. 'What Size????" he said.
To get them from the US or Europe Ebay/Amazon costs a fair bit more than the tyre in freight.