Roundworms, i believe, are from eating raw meat (birds and mice), but I could be wrong! I know that all kittens are born with roundworms (don't know why or how true this is but have heard it repeatedly and know that vets treat all kittens for roundworm when they get first distemper- at least around here...)
Well, be glad you didn't see fleas!!! That's a good thing. I hope you used a flea comb or fine tooth comb to check for this because the flea gets trapped between the teeth of the comb, so when you stroke a couple times, then check both sides of the comb, you'd see a flea and/or eggs which are black, (called "flea dirt"). The eggs will bleed if you drop the comb in a bowl of hot water.
Hope you had luck with 2nd vet or able to get a stool sample (why is that hard for you to get?). keep us posted!