I need to implement a backup solution to backup my office users outlook PST files.

these are archive PST files that my users archive every now an then.

are they any suitable backup solutions in the market that can do the job ??

PST files are usually "locked" when the outlook client is open.

Yes, the .pst file is locked and cant be copied when the Outlook client is calling it.
Based on my experience, the most simple way to back up the PST file is to guide the user put their PST files to file sharing server which is based on per-user permission.
As a IT admin, we may also script a code or use tools to collect the users PST file from the local default path. Notice your co-workers to turn off Outlook  and keep the network connected at the certain time before you do that.

Anyway, if there is any better solution from the other vendors, contact them might as well.

Tony Chen
TechNet Community Support