A site like Firenzi MVSEI's webshop.b-ticket.com for Venice, Rome, Sienna attractions?

The b-ticket site is the official ticket vendor for many of Florences museums whereas Rome is a little different. The best ticketing resource for the Vatican is its own website:

http://www.museivaticani.va/content/museivaticani/en.html

And the Colosseum/Palatine/Forum + some other attractions:

https://www.coopculture.it/en/

Galleria Borghese and Castel S. Angelo:
http://gebart.it/en/home_en
(This is a portal to the ordering pages)

These are the attractions you really need advance tickets for to skip long ticket lines, although possibly not Castel S. Angelo.

Additionally, there are tours of some of the above which favored by a fair number of RS travelers to Rome and are available through private companies, such as "Walks of Italy" and "The Roman Guy" so short story long, you're going to need to use multiple resources for Rome depending on what you want to see and HOW you want to see it.

For general-entry tickets, I wouldn't use any 3rd-party sites which could have you paying more than you need to.

The official booking site for the Borghese gallery is

https://www.tosc.it/tickets.html

You must book tickets because of the limited number of people allowed inside at a time. You won't skip any lines with your ticket, it just guarantees entry to your time slot.

For the Castel Sant Angelo, just buy onsite. There is usually never a line.

For the Vatican museums and colosseum use their official sites for entry tickets or tours:

https://biglietteriamusei.vatican.va/musei/tickets/do?weblang=en&do

https://www.coopculture.it/en/colosseo-e-shop.cfm

I don't use third party vendors when I can book directly. Also, if you book far enough in advance you can get great inexpensive tours directly through the official sites without needing to pay private companies their very high fees.

Donna

The link to the Borghese I provided above is a portal to the https://www.tosc.it site.

Each city is different, as well as not all the top sites are every really organized into just a single source. For the number of cities you're covering, I hope you have one of Rick's guidebooks that address these details. Venice, for instance has a number of sightseeing pass options, some of which are at http://www.veneziaunica.it/en/. But before you buy anything I would read more on pages 61-62 and 72 (pages from Italy 2018, not Best of Italy), for a start. Each museum and church also tends to have its own web site.

Once again thanks Kathy!!!

And thank you too Laura! Yes, I do have Italy 2018. Some reason could not find info at first. Thanks for info and page numbers.