thanks for the pics. that is a tfx psu. an odd size for sure. this is the best one i see on newegg that's a direct dell replacement. it is 400w and only $30
https://www.newegg.com/replace-power-rp-sff-l250ns/p/1HU-009N-000B8
i did forget to ask though why you wish to upgrade it. what do you want to run that needs more power. this is just to be sure 400w is enough power. it does not have any pcie connections for graphics cards either so that still limits what gpu you can put in it as well. something that only needs mobo power is the best you can do.
edit i did find this one on amazon that has a 6-pin but i know nothing about the brand or its quality.
Buy Athena Power 80+ Bronze 400W TFX12V Power Supply (AP-TFX40): Internal Power Supplies - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
www.amazon.com