Another good option are DIY markets and special professional stores for painters or carpenters, e.g. from buying co-operatives. I do not know if you have something like the latter in the US, but these have a much bigger assortment of material, especially for high grits and other surface preparation stuff you can use for bowling.
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DizzyFugu --- Reporting from Germany
"All that we see or seem, is but a dream within a dream..." - Edgar Allen Poe