I don't think you can order it online - it is a vet prescription medication. Your vet will likely want to use Panacur or Strongid. If they want to use Drontal, tell them you've heard a lot of round worm have become resistant to it.
I hate to say it sweetie - but if your other kitties are indoor cats, and Ditto has the round worm bad enough to be throwing up worms, they most likely have it as well.
To treat the round worm in our home, we had to use the Panacur on ALL six cats at the same time. It was NOT easy. Before this, we did each kitty individually as it became clear they had worms - and it took us over a year to get rid of it. And we didn't get rid of it until we did ALL of them at the same time. And the life cycle of the round worm is such that you have to re-treat ALL of the cats three weeks after the first cycle of treatment. The Strongid, Panacur or Drontal only kill the adult worms - so you have to wait for the eggs to hatch and become mature, and then give another round. And, unfortunately, the de-worming medication sold at pet stores and supermarkets doesn't work.
While you are giving the kitties the worming medication, you must keep the litter boxes super clean. I mean - scoop twice a day. And the boxes pretty much need to be dumped of all litter, bleached, and have new litter put in weekly until three weeks after the last round of medication is given to the cats. At that point, you should take Ditto's stool sample in to the vet to have it checked for signs of round worm - to make sure it actually is gone.
It's a pain in the butt, but a few months from now you'll probably regret it if you don't put the time and money into it now.
Like I said - we fought this for over a year. Please learn from our experience!