The key principle to remember in any collection matters is that you should not throw good money after bad.
That being said, the companies that can help you find the old tenant/missing person are called Skip Tracers. Google this term and you will find related companies.
If you find the ex-tenant and he/she is unwilling to pay, you can commence enforcement proceedings. This can be done in a variety of ways: garnishment of wages, writ of possession, writ of sequestration etc. Since you had received the judgment 8 years ago, you may have to seek leave of the court to get these writs issued because usually these writs are valid for 6 years - it would depend upon your particular jurisdiction though.
You can accomplish all of this on your own, but you would need to spend some time to understand these processes and these are unfortunately not straightforward especially if you have no legal background.
Please do consider whether it's worth your time and effort to pursue this.
For enforcement in Ontario, you can also check out this link for details on the information I have provided above: https://www.lawhelpontario.org/lawsuits-disputes/superior-court/how-to-guides-superior/guide-enforcing-order/