@caz Even though they use proprietary pinouts, the connectors themselves are still industry standard. Computer parts stores may have adapters if you search for something like "dell <series> <modelnumber> to atx psu adapter".
Alternatively, stores may have NOS or even new compatible PSUs if you search for the exact model name/number. (Make sure they don't send you a standard-ATX instead if you go this route.)
@caz That sucks. The last time I had to play Dell's "don't fry the motherboard" game was 15 years ago, for a customer's Dimension 8100.
Even if a ready-made adapter doesn't exist, it shouldn't be too terribly hard to get Dell's pinouts for your specific model and make your own adapter or have a custom cable shop make one for you. I found these diagrams, but I can't vet them: https://pinoutguide.com/Power/#Dell