Like private IP address ranges (defined in RFC 1918), there are also private MAC address ranges. These are called Locally Administered Address Ranges which are never used by devices or other vendors. MAC addresses in these ranges can be safely used, assuming they are unique within your network:

x2-xx-xx-xx-xx-xx

x6-xx-xx-xx-xx-xx

xA-xx-xx-xx-xx-xx

xE-xx-xx-xx-xx-xx

The ranges may seem odd but is explained best by this excerpt from Wikipedia:

Universally administered and locally administered addresses are distinguished by setting the second least significant bit of the most significant byte of the address. If the bit is 0, the address is universally administered. If it is 1, the address  ...