Question about Oral-B Pulsonic Slim

I have an Oral B Sonic Type 3737. Salespeople from Coop, Interdicaunt and melectronic say that they no longer receive toothbrushes for this appliance and that Pulsonic toothbrushes do not fit this appliance. What is true now? Is there really no stock left? Do I have to dispose of the device that still works? Where is Braun's promise of sustainability?

I have both ("big" Oral-B for home and the Slim for on the go). The brushes are not compatible with each other. They have different/longer connectors: https://photos.app.goo.gl/3cl9y6nGjD...