There is a common belief that the individual plastic tensioner is unavailable and can only be sourced either as a second hand part or through the purchase of the entire assembly.

This plastic assembly appears to be available through a Wheelchair conversion manufacturer.

The requisite part numbers are...

FP4841 DOOR OPERATOR CABLE TENSIONER for one side and

16645318 DOOR OPERATOR CABLE TENSIONER for the other

It looks like they are available through a company Braun and have a website BraunAbility.com

Whilst they do not list the parts on their website as of June 2021 there is a listing on at least these two locations.

http://949vans.com/VMI-2008-to-2016-...oor-Parts.aspx

and

https://donbrownbusparts.com/product...5318-rt-e51173