NRS sized frames uses the 1.25" version and some DRE and other frames with the larger diameter tubing use 1.5" fittings.
Downriver is Colorado's largest Holleander fittings dealer and at least for their normal t's and such they have been very reasonably priced and comparable to a lot of what I found online. They sell a similar umbrella setup too though, so they'll probably just want to sell you that. It will get a bit harder if you want a fitting that is one size in one direction and another size in the other. Probably easier to make an adapater or sleeve that sizes down the tubing to fit the umbrella.
This would probably work for your needs... https://www.buyrailings.com/product/...s-assembly/769
p.s. To avoid confusion... most raft frames use schedule 40 pipe (usually aluminum) not tubing. The only difference you need to worry about is that pipe is measured by the inside diameter and tubing is measured from the outside. This is why you would buy a 1.25" fitting to use on material that measures 1.660" on the outside.
p.p.s I use a NRS lopro fitting and some 1.25" pipe on my frame to mount my gopro onto...and it makes an admirable umbrella mount. The shaft of the umbrella fits inside the pipe and sits at just about the right height to sit on the boat and be covered. I don't have the pipe bolted to the lopro just in case I flip or something, but have it tied on with a lanyard. If I flip, its loose enough to just fall off.