If ever a thread should've had my name on it, that was the one.
I love Sirop de Menthe. I used to drink it all the time as a kid. With water, or lemonade, in the summer, it's just perfect.
Anyway, there are a few places I know of where you can get the original Teisseire brand, like in France. One is right next to Baron Court's tube station. If you exit the station, just on your right is a delicatessen kinda shop, and they usually sell it there.
Otherwise, there's a Costcutter on Blythe Road (near Hammersmith), where they sell it, as well as a ton of other French stuff, like yoghurts and biscuits and compotes and stuff. Costcutter are franchises, so they each choose what they import, and that particular one is totally french. Awesome.
But if you wanna play it safe, you go to South Kensington, and check the shops around the French Lycee and on Harrington road and Bromtpon road. I'm 99% sure there's a cafe on Brompton road, opposite the Tesco express, that sells it.
Good luck!