There are many, here are the main ones in no particular order. I have dealt with all of these and had positive experiences otherwise I would not list them. It’s always possible your experience may differ.
Alan Wheely in Toronto is an authorized SIP seller. If you are not in a rush, you can contact him and get in his next parts order. Link to him on our garage resources blog post. Alans Garage
Scooter Fix just outside Ottawa is a Lambretta specialist with a small amount of Vespa parts. The owner Corey keeps has a good supply of mostly stock parts on the shelf. He also puts in regular orders to several of the large parts suppliers if there is something you need that is not listed on his site. https://scooterfix.ca/ . In Toronto, I find Canada Post regular mail delivers in 1-2 days.
ScootArt in Montreal is a full service garage and scooter shop. Has both modern and vintage, Vespa and Lambretta. He has some parts on the shelf and puts regular orders into the large sources if you want to purchase through him. JF the owner, seems to like odd-ball other manufacturers, so you can always try him if you need parts or work done on your Heinkel, Zundapp, etc. http://www.scootart.com/
SIP Scooter Shop: Based in Germany, has both Vespa and Lambretta parts. Large and professional. If you order direct from them you can select an option for a few extra Euro’s where your order is packed and couriered in one day. My experience with this in Toronto is I have my parts in 5 business days. https://www.sip-scootershop.com/en/main/base/home.aspx
Scooter Center The other big German outfit. Has both Vespa and Lambretta parts. Large and professional. https://www.scooter-center.com/en/
Scooter Restorations: Large Lambretta specialist in the UK. http://www.scooterrestorations.com/
Jet200: Lambretta specialist in the USA. Stocks a lot of performance parts. They also have the tooling to straighten damaged forks and frames – but beware their turn around time on this service can be slow. www.jet200.com
Scooter-Speed.com: Has both Vespa and Lambretta parts. Lots of performance parts. http://scooter-speed.com/
Scooter West: Carriers new and vintage Vespa parts in the USA. https://www.scooterwest.com/
Note, there is a long list of small international shops in USA, UK, Italy, and Germany that make exceptional parts or provide great service. Sooner than later I will add them.