EU says more are born... Palps scoops some up. Maarek Steele, Mara Jade, etc., and they fall into different roles for him. There's a whole "inner circle" of sorts of force users that either he, or Vader, or both wind up training.
I think Yoda/Ben look at Luke and Leia as the last hopes due to what they assume are their natural force abilities. Maybe they tested them as infants? Also I think having nobody to really train them to fight two seasoned and very powerful Sith would be a big roadblock too. Vader's obviously super powerful, and Palpatine took Yoda to something of a draw. Throw in their political/military power. Anyone Yoda or Kenobi trained would be in for a fight unless they had inheritly natural strength I guess.
It probably also takes work to find them... many going unnoticed. Yoda and Kenobi would be exposed looking for them I suppose.
I can't think of many other force sensitives at this exact moment but I know there were others. There were other trained Jedi lurking around too who survived, if you buy into EU of course. Shaak Ti seemed to escape death a lot, Rahm Kota, Qu Rahm (I think that's the spelling) as well, at least for a time... Till someone got 'em. Then there were the Dark Jedi, like Jerec and his group of weaker Dark Jedi (Sarriss, Boc, Gork & Pic, Yun, and the guy cut in half on a floaty prosthetic whose name I forget offhand)... I guess Jerec was off in wild space while the Clone Wars went on and he got goofy out there, and came back armed with some lesser apprentices and took over some of the Imperial remnant and killed Qu Rahm in the process... though when he kills Rahm in the storyline's tough to tell, but it's into the Imperial era as Rahm appears to be helping the Alliance, possibly like Rahm Kota was in its earlier days.
That's EU though... take or ignore at your leisure. That's what I do.