I get mine from Mondo's butcher in Inglewood...

But I think you can also get in at Simon Johnson's in Subi.

The best brand (by far) is called "Terrabianca - Olio aromatico al tartufo"

It's only a small bottle...tall and thin...but it lasts for ages...you literally only put a few drops on for each serving...

Probably best ringing either of those places just to check that they have it in stock...