I am not aware of any US dealer at all that has the Tommaselli clip-ons, they from what I gather seem to have been a vendor that is well known and associated with the Moto Guzzi aftermarket scene more than anything else.
For some bits and pieces you will find that you will have no choice but to go with suppliers outside of the US because they just cant be found here in the US.
If you are still interested in getting the Tommaselli's and are willing to look across the pond, give
www.motomeccaspares.com a shot, there shouldnt be any issues.
Dont forget for '05-06 S3's it's 52mm.