Parts diagrams, etc.:
https://manuals.wotmeworry.org.uk/Vespa/LX150/
This I believe is a prominent member's comprehensive repository…not doxxing him in case there's a reason it isn't readily attributed.
Parts: is this a trick question? Discussed all the time. My personal go-tos are Scooterpartsco, Scooterwest (which was the first link when I googled "lx150 piston"), AF1Racing, and SIP-Scootershop in Germany.
If you're messing with the LX piston you might consider upgrading to the Malossi 190cc kit, which I see Scooterpartsco has. Robot at Scooterwest put that in his LX as one of his many "dream scooter" videos.