I see - I think you could just scratch off bespoke and lean towards MTM places where most will be more than happy for you to send over your trousers to get copied. Although I'm in the UK so wouldn't be able to recommend you any brands at all apologies.I'm in the US not too far from NYC.
I'm not necessarily interested in the rigamarole of the full bespoke process, just want to send these trousers (which are OTR but fit perfectly) in and have them copied in a couple of different fabrics.
Are you planning to choose fabrics online or actually buying the bunches to send?