You didn't say b/w or color.
Most common mat I use is Crescent 1607, Bright White, an acid-free rag mat. We use something different when warm/cool art doesn't agree with the C-1607, or if it's a series we have to decide whether we can use the same color on all or not.
If you don't have the luxury of putting samples next to your photos & splitting hairs over which white to use, that's what I'd suggest. Mail order would be your cheapest way probably, but please don't go to your local frame shop to use their samples, then leave to place a mail order purchase :O(