Is there a replacement mailbox lock that is more pry-resistant?
I believe you're going to find that the lock itself is not the weak link in this mailbox product.  The doors are thin Aluminum and I believe are easily pried open with a standard screwdriver or similar tool.  You can replace the lock with a high-security version if there is such a product available for this model, and you still will not solve the problem.
I have a couple of suggestions, however:

There are higher security mailbox systems available from numerous vendors.  These are more expensive that what you have but having your mail stolen can also be quite expensive.  So replace the current flimsy unit with a better one to discourage opportunistic theft.
Relocating to a more visible location, adding lighting, enclosing the mailboxes in a structure that only residents and the USPS have access to might be helpful.