You can look up Songmics, they offer all kinds of systems mostly for household use, but I could imagine that some of them also fit in a transporter. It's then considerably cheaper than Sortimo, but probably not as well specialized for craftsmen.
With common cars/vans, the used market also offers a lot! We installed used Alu modules in the T6. It fit perfectly for us. It's difficult to give concrete recommendations because the requirements are simply too different (do you need compartments, boxes, drawers, long shelves, etc...)
Sortimo, Bott, Orsy and co. don’t differ much, they are modular and reusable and eventually surprisingly price-stable
I am fairly sure that there have been legal adjustments recently (as is currently the case with ladders in commercial use) that cause/have caused not everything to be permitted anymore. The area "connection of the transport boxes/shelves etc. with the vehicle" must have been affected. For commercial use, it could therefore be that used goods are no longer allowed (exactly for this reason, it is probably "becoming available" right now).
I could be mistaken, but I usually keep such things roughly in mind. So inform yourself about this before purchasing, otherwise you will have spent money and still have to undo it afterwards. It must have been mentioned in some magazine (something from the [IHK] ?) that most companies are mandatorily delivered to their premises.