Howdy neighbor! I have a pretty good list of online venders I've used over the years, and only one of them had ulva in stock - Blue Zoo Aquatics. Now that I've had it for a couple weeks, I find it hard to believe so many venders were out of stock. It grows insanely fast. I had singled it out, along with chaeto, as the best fuge macro, for nutrient export and pod habitat. I've never seen it in any shops here in Boulder.
This was the first time I've bought from Blue Zoo. They're not the cheapest vender, but they have quality stuff. I was also able to negotiate shipping down from overnight to 3 day, so that helped. They're good people!
Anyway, I bet other venders have it in stock by now. Google sea luttuce or ulva, and it should turn up. Good luck!