We include the microSD card and fully test each one to alleviate compatibility issues.
If you lose it and If you want to use your own, try any 8, 16 or 32GB card if you have them. Class 4 is good enough. Format Fat32. Firmware and log files are small!
If you want to purchase ours, you can find them out here - https://emotimo.com/products/replacement-micro-sd-card-holder