not sure how you get 240w in 5090 sff.  standard Dell spec for 5090 psu Tower 260 W typical 85% Efficient PSU, 80 Plus Bronze 300 W typical 92% Efficient PSU, 80 Plus Platinum 360 W typical 92% Efficient PSU, 80 Plus Platinum Small Form Factor:300 W typical 92% Efficient PSU, 80 Plus Platinum 200 W typical 85% Efficient PSU, 80 Plus Bronze a 240w psu is not included in list above. J82T5 is 300w psu for XE3 which has 6+4 pin white for MB + 6 black pin for gpu.  It is not for 5090 which needs 6+4+4 white for MB. you can get 300w psu for 5090, model number H300EPS-00, DP/N V2KRY.  Order directly from Dell since eBay is out of question. you can also get Dell 360w psu for 5090 tower which is c/w SFF too.  6+4+4 pin. the highest you can get is Dell 500w for 7090.  Those have 6+4+4 for MB. 7090 =C65R4 500W LiteonDG3PM 500W ChiconyXJ5VJ 500W Delta chat w Dell sales online for pricing and availability  if you can buy on Aliexpress they have Dell 500w psu too. PS 240w psu is okay for gt1030 in 5090.  The 300w recommendation is generic round up estimate.  more accurate psu calculator estimates load wattage which is max power consumption in typical real world scenario < 240w.  more like 204-212w.
I've got an Optiplex 5090 with the 240W power supply and I'm looking to add a video card and additional ram for some decent business needs. The RAM is nbd, but even looking at a MSI GeForce GT1030 it's recommended I move to the 300W power supply. Would the J82T5 be the correct option to do so? Also, what is the best route to purchase? Ebay is a no go with our purchasing restrictions. Thanks!
Optiplex 5090 SFF Power Supply