I have an Ultegra 5-bolt Triple Crank with a 10 speed cassette, and would like to replace the small 30T chainring with something in the neighborhood of a 26T ring.

Anybody have any experience with this that could recommend a vendor for the chainring?

You need one with a 92mm BCD. I'm not sure anyone makes one...

What's your cassette range?

Only one generation of Dura Ace used the 92mm BCD inner ring, which was only available in 30T. More likely the OP needs a 74mm BCD, such as: https://sheldonbrown.com/harris/chainrings/74.html#26

Do a little research before posting.

https://techdocs.shimano.com/media/te...9830676705.pdf

Didn't realize the 6703 Ultegra also used the odd-ball BCD. However, the 6603 model used 74mm and the 6600-series of Ultegra components were also 10-speed. OP: can you clear up which you have?

[Edit. Got rid of my options because davidad's is much better.]

26 lol

If you have the ****mano 6703 with the 92mm small ring you will have to get a TA adapter middle ring that will allow you to use 74mm BCD rings. May as well go with a 24t just in case you ever need it.
The 6603 uses a conventional 74 BCD ring.

I have the Ultegra 6603. Does that mean a 74mm BCD/26teeth specified for 9-Speed will work for me?

Some would disagree, but many would say it is okay to mix specified speeds of chainrings.

Yes, it will work. I've made the exact same change on numerous 7,8 and 9-speed Shimano triple cranks and even on a 10-speed Campy triple crank and they all work well. Generally, I've used generic 74 mm bcd rings as granny rings aren't pinned, ramped, etc the way the larger chainrings are.

I would recommend adding a "chain watcher" to assure the chain won't spill to the inside on an emergency shift under load. Shifts to the granny ring are often done in a real hurry after you realize the hill is a LOT steeper than you thought.

The 24t and larger will work.
The shift up to the middle ring won't be as smooth, but it is still easily done.

Yes.