This is a perfect opportunity to go back to your A-B distributor and ask for help.
Maybe your company has been so bad about paying for support in the past that they'll tell you to buzz off unless you want to pay for a full-scale support contract.
Or maybe they'll take this as an opportunity to try to sell your company on the benefits of TechConnect or some of the other service products that Rockwell provides, and find the support contract that fits both your needs and your budget.
Or maybe they'll take this as an opportunity to just get in the door a little bit. An old PanelView firmware file doesn't cost them anything. But a guided tour of the facility and a couple of hours to discuss what sort of plans for migration, upgrade, and maintenance you have of your automation equipment is how distributors and salesmen build relationships.
I did find a list of the PanelBuilder32 / firmware bundles in the RA Knowledgebase as
Article ID 34399 (Access Level:Everyone). It looks like firmware version 4.20 was included in the PanelBuilder 32 version 3.71 and 3.80 releases.