The greatest potential fuel source is spent reactor fuel.  The enrichment is lower than when first inserted, but still much higher than natural uranium.  In many cases, take out the fission byproducts (everything below Uranium) and just stick it back in with some new uranium nearby (perhaps enriched a bit higher).  Plutonium, Americanium, Neptunium are all fissile.

Not going to happen in US, but I bet China would do it if need be.  And offer to take troublesome nuclear waste off of others' hands.

There were a long series of posts on the Energy Resources forum (somewhere between 1-3 years ago) arguing that U could be extracted from the ocean by suspending nets.  I don't remember any specifics but I think the nets were supposed to be treated to "attract" the U - but don't hold me to this :-)  In any case, the arguement was that there was unlimited U that could be extracted.