I am Canadian and live in Canada but I grew up in Cortina and still spend some time in the area. Although I don’t have any direct contacts for Bike rentals (Skiing is my business over there) I found this with a simple Italian search. These guys are past Belluno but in the area.
https://www.noleggio.belluno.it/noleggio-biciclette/ You can also just use the search “Noleggio la Bicicletta” and add “BIci da Strada” is road bike. You can also attach Belluno, Treviso (if your coming from Venice) or Agordo (Home of Luxxotica eyewear which own 80% of the worlds glasses production including Oakley) which are some of the larger centres in the area. Also if your in Colorado, maybe contact the guys at Cinghiale. It’s Andy Hampsten Cyclo tour company. They are located in Boulder. And they run most of their tours in the area. If your trying to email anyone there. Google translate app helps. Not perfect but you’ll get your point across. Also, most speak some English there. So you can always try in English first or as well? I hope that helps.
Mike