Hi, I bought a 3rd party tripod mount for my 70-200/4 IS for a lot less than the original part and I'm happy with it. I don't use it too frequently so I can't really judge durability, but on the other hand it's only a piece of aluminium regardless of you buy the original or a third party version. (Maybe they're actually manufactured at the same place but sold under different brands - I don't know.) I bought mine on e-bay but I dont remember from where. There are several sellers of this product on e-bay - just pick one you feel comfortable with.

PS. The tripod mount doesn't fit my macro lens (100/2.8 L IS), maybe the non-IS version is slightly smaller. DS.