I'd say transom is best if the dinghy has one. An inflation valve could work if you cut out some parts, but they are fairly tall and you would either have a significant projection below the boat, or if installed from the other side, you would only have access to unplug from below the boat.  I'd be tempted to go down to the hardware store and look at what they have in the PVC pipe section.  You'd want a threaded fitting you could cut down with as wide a flange as you can find to grab the fabric.  If you have a PVC dinghy then you can even glue it in place.  If I couldn't find that, I'd go with the inflation valve David suggested, but cut it down aggressively and just use a rubber plug rather than the plug that comes with inflation valves.

Iain Grigor wrote:

Does anybody have any ideas about how to source and

install a drain plug in the (floppy, rubber) floor of an

inflatable dinghy?

Does it have a hard transom? That is where the drain plugs usually are for inflatables. Otherwise I imagine you could take just about any kind of drain plug and insert it through the floor, (hull) so long as you reinforced the hole you have cut, maybe even with a small round of plywood glued onto the hull fabric.