Finding a Dumble for sale is challenging, even when money is not an issue. You can look on https://reverb.com/, where occasionally you might see something pop up. If you live in an area where you have access to a proper Vintage Guitar Store, you can ask them. Sure, they might laugh in your face, but they are probably the only people that would know if there’s a Dumble in the area. Anyway, I found one Overdrive Special for sale in London (UK). It looks like Joe Bonamassa was the previous owner of this Dumble, so you might have to pay extra for that, but anyway here’s the link: https://www.denmarkstreetonline.co.uk/product-details/dumble-overdrive-special-combo