A RC servo can simply consume current up to 2A in stall condition. In C40R case, I would estimate 1.0 to 1.5A of current needed for good operation.
6V is the maximum voltage allow for C40R. A normal 7806 regulator can do the work but a 7806 regulator can only regulate 1A of current maximum and it will be extremely hot for long time of operation, furthermore, the source for 7806 must still need to have enough current. In simple words, if you need 5A at the 7806 output, you will need to supply at least 5A to the input too.
Therefore, to get yourself 5A or more power supply, I can recommend a few options:
1. Battery such as
SLA 7Ah 12V2. Switching power supply,
12V 8.5A3. Or get yourself ATX power supply from computer store and uses
BB-ATXRA to obtain the 12V. You can try the 5V too if you want.