northeastcoin.com (NEN) has one of the biggest selections. Although keep in mind the great of majority of slabbed world coins reside in NGC holders, they have been doing it for many more years. Our own Civitas usually has some slabbed although he carries a lot of raw material, Aspen Park usually has a good selection, if a little pricey it's nice stuff, NFC usually has a collection of slabbed raw although I have not bought from them yet.
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World Collection
British Collection
German States Collection