@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
@arielmt Thanks! Unfortunately the only website that had my adapter available and in stock was so broken it kept getting my billing information wrong. Glad to know an adapter exists somewhere on this planet though.