It depends on your country but I would have to recommend our company www.gpslsolutions.com 
We are all long standing XML geeks and have offices worldwide (HQ is in UK). We know how to navigate PTC and have done many Arbortext implementations.
Of course you can also lookup resellers via www.ptc.com -> Partners -> Find A Reseller. Just be aware that some resellers who are marked "Arbortext" may be IsoDraw specialists and not know much about Editor. That is because ISODRAW got rebranded to Arbortext a few years ago. Also some of the bigger Windchill resellers are marked as Arbortext because they want to grow their business (even though they have no Arbortext skills).