I'm a bit surprised that so many seem to be aligning with the LGS rather than the perspective of the OP. Seems to me that he is a customer in good standing for more than two decades. Has gotten to know the staff and management and has brought many times his own purchases to their firm via referrals and the like. So, as before, when he asks to have a small part ordered, as before, he should continue to expect that no downpayment (in person no less) is required.
I understand that their policy may have changed. And for new customers, so be it. But it's well known in business that the cost of maintaining a customer pales in comparison to the cost of acquiring new customers. That's why in my business (enterprise software/hardware) when a good customer needs a shipment and their business depended on it, we'd put in on the trucks and pay overnight Fed Ex air to ensure their business continuity. This could be a repair, some upgrades or add-ons, new software licenses or whole racks of loaded servers and storage. It's value could be in the millions. But so what? They're a good customer who depends on us. Some weeks later when the urgency subsided, the account owner would meet with them to figure out how much it all was and when it should be payed. No big deal. You do this for your good customers and it has to be a key difference between face to face transactions at a LGS and internet dealings where these things are a lot harder to support.
So, if I were the OP, I'd calmly meet with the manager, ask for his/her perspective and remind he/she of his long-standing relationship with the LGS. If the answer is a dogmatic 'it's the new policy', then I'd call that relationship done with, and only shop there if or when they had a lower price on some desired item. No emotion, no demands. Just understanding the new rules by which they want to play and how it will affect their business and the customers behavior henceforth.
B