Have you tried searching in the supplier's directory? Alternatively you can email email@example.com to help you find sources. Or if you have considered importing from China, if the quantities are right, we can help you source these directly from China.
It is a good idea to carry out due diligence as SD Cards is an industry that sees more fraud in China compared to other industries because this is a high volume - low margin product and new importers fall for "highly attractive" below market prices and end up with below capacity cards. The wafers for these cards are normally made in Taiwan as the wafer technology in China is still not yet up there.