Richter Precision in East Petersburg PA does DLC, as well as a variety of other PVD, but parts need to be sent in from an FFL. They will not accept from an individual, regardless of whether it's serialized or not.
You send clean, disassembled parts and they will prep and coat them. If the parts are not disassembled they will ship it right back. Pricing to the FFL is reasonable, but obviously the dealer is going to mark it up. Minimum order is $150 if I recall.
If you cannot find anyone else to do it, might be worth making friends with a local kitchen table dealer and have them send parts to Richter.