I just installed a brand new cover last week, and filled the tub back up this past weekend. I did a ton of research and looked at old posts on here.
I ended up going with rhtubs.com as they were mentioned in several posts from other members who have used them in the past. They delivered it to me 2 weeks sooner than I expected which was a nice bonus.
I also went with a different material than the standard vinyl (Weathershield fabric), as our tub gets cooked in the California sun and sits near some windows that reflect the sun onto the top. The new cover I received is a very nice cover and much better than my last stock cover. I will try to get a photo and post a before and after...
I also looked at thecoverguy.com, nashvillespacovers.com, arizonaspacovers.com, hottubworks.com, and spadepot.com - but eventually decided to go with Roberts Hot Tubs as they seemed to offer better options. I do think many of the cover sellers use the same actual manufacturer or suppliers.