Optiplex 5090 SFF Power Supply

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 Liteon
DG3PM 500W Chicony
XJ5VJ 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.