I buy them in packs of 50 through local IT distribution channels in Australia. I think it was via Ingram Micro, who are also in the EU. The margins aren't very high, so there isn't a lot to save on 50. You should be able to get slightly better pricing for 500, but that is still a reasonably low quantity. I just googled the part number and contacted some of the sites stocking them.
I've stuck to Team Group cards as they seem to be less likely to be counterfeit compared to Sandisk or Kingston, although using a reliable distributor should reduce any risk of grey market parts. I've used 2GB Class 4 or 6 in the past, and now use 8GB Class 10. This is for custom dataloggers (msp430/STM32), and on some products running embedded Linux.
The one time I needed a quantity of 500 for a production run, I specified the brand and part number, and left the sourcing and duplication to the Chinese factory handling the production. I saw they used a PC with a 32 port USB hub and 32 microSD to USB adapters, but not sure what software they used.