Your Vendor/Support company should be able to provide it.
The version you are after is the 6_0_9_57-DBP_Plugin (the numbers & versions don't quite tally up)
I believe you have to be a registered Mitel technician/partner in order to get to the downloads section on the Mitel website so your best bet is your vendor.
Like I said, I've tried that. They were unable to help. Trying to find an alternative download website or something. I got a feeling I'm going to trash this system and get Asterisk.
Is this the vendor you purchased the system from? The one who hold the license application record and are you in good standing with them (meaning, you don't have outstanding debt that is not on contract or mutual terms with them)?
If so, please PM me the name and location of the dealer, and I can get you in contact with their regional Mitel representative that can put some pressure on them.
Now, if you purchased this system used then unfortunately all bets are off... You need to get the application record moved to an authorized vendor, which is NOT a free service. This is not an Asterisk platform, it is not "free", all users and some services require paid licensing. If you cannot get an authorized representative from the original owner or vendor to sign off on it and release the application record to your vendor of choice you must purchase a new one.
Without more information I cannot assist you further...