To clarify, limitations on media card size is not based on the phone but on the OS. This is one of the items covered in the great FAQs here on Blackberryforums.com. What type and size Media Card can I use in my device? - BlackBerryFAQ
I usually buy through Amazon, either sold directly form Amazon or from one of the marketplace vendors. Like ebay, buyers should check out the feedback and ratings for any vendor. I've never had problems with any that I've used. Frequently, you can get the san disk cards in "bulk packaging" which is cheaper than retail packaging. Means you just don't get all the extra cardboard you need to throw away.