As topic, any ideas where I can get it from?

Never seen any, you could use an overpaintable underbody shutz, or stonechip. Then finish with whatever colour you want.

Gonna do the arches and a few areas on a 55 plate transit Ive just got hold of and wanted to keep the OEM look.........

What you need to ask for is stone chip which comes in grey, white or black. You can get this from Halfords in aerosol form or from most paint factors in a can that screws onto a special gun. The guns are not expensive but you will need a compressor.

You want 3M body schutz as R1_pete says and paint over it with the same colour as the van.