I would hit that 'Brand Loyalty' thing by making certain things by certain manufacturers more difficult to get a hold of; NeoNET (or its subsidiaries) doesn't do many cars, for example. Also, tweaking a couple things up (and down) by 1 should be good -- maybe Renraku's knock off of an Ares weapon is a little less accurate (but a little less expensive), or MCT has perfected the steering systems for a drone (and made it a bit more expensive).
You can also make an alternate harder to get. He hates AZT, but AZT has the market for That Item cornered in this area. So getting an alternate might involve -- ping! -- a run!! And it just so happens that his armorer has a line on Alternate Manufacturer trans-shipping a bunch of That Item nearby, and if you liberate a crate, he'll buy the rest ...