Volvo dealer can source it too. I have the P/N somewhere, or they should still be able to look it up. Last time I think I bought 5 of them for ~$30 or thereabouts and didn't have to wait too long, not bad. Don't use RTV on it, they seal up well with a fresh gasket, or frequently even with a good used one.
You can also get away with using a thin-section large circular O-ring instead of the proper oval one, and in fact the aftermarket head sets you can buy for these come with that variant -- only big downside with that, besides finding one in the right size, is it's harder to keep it in the groove for initial installation, have to use some sticky grease and even then it can be tough if you're working on a 240 with the engine tipped further over towards the pax side, or worse still if it's a Pinzgauer with the engine laid over even further (as is probably the case with an LT also). Proper part from Volvo or VAG probably the least hassle.
Now, on to the more important question (@BusBoyBob): you've got a diesel LT28 here on the west coast of the US??