There are lock picking kits and tutorials on Ebay, for a few dollars, that Ive had success with.
When that failed, Ive either some success with locksmiths.

On a couple occasions, Ive visited our East coast run of Antique shows, where there was always a few people
with boxes of keys for $1 each. I found a Picture online that I worked to match general shape, and have gotten lucky a couple
of times. Even found a key collector, who dug through his stuff, and found one in 10 minutes....other times Ive spent a day and failed.

I hunted for a replacement, before I drilled out....

Post a Pic, and maybe someone here can help describe the key