Yes....
Any over the road truck dealership. Auto shops only go to 3"-3. 5".
You just need a turn down? I mean, that's all? You'r adding one to existing tail pipe?
I guess sometimes I think everyone should know what I know, but if you'r not familiar with Diesels/OTR trucking you probably wouldn't know where to obtain the larger dia. pipe.
They will have all types/sizes of turn downs. Yes, in both clamp on or weld on. Either way.
Hope this helps, unless I missed you'r point entirely.
Scott