How can i get a PSU 300w for my Dell Vostro 3710 SFF

Anything other than the original power supply will need to be mounted externally to the case, and will need a 24 to 8 pin adapter cable (assuming someone has made one that will work with this system).

@ejn63​ So you tell me i have to buy a PSU and the cable and put it in my pc?

No, you cannot install a 300W power supply inside the system.  You'd need to leave it outside the case and route the cables inside.  The main issue is whether or not you can find or build a converter for a standard ATX power supply to the 8-pin Dell mainboard connector.  Last notice I have seen is that there aren't any for this system.

So you tell my cant use a 24 to 8 pin?

3710 uses new Dell proprietary 8 pin for which no proven safe adapter is available off the shelf.   the commercial 24 to 8 adapter is for much older pc model.

you can however use Dell oem 300w psu for new Optiplex 7000/7010 SFF in Vostro 3710.  both systems use same 8+4+4 pin from psu

this 300w psu will fit the 3710 case interior.

https://its.weill.cornell.edu/sites/default/files/7000_tower_sff_and_mff_optiplex_data_sheet-3-15-22.pdf

Dell offers 300w oem psu for 7000 SFF.

https://www.dell.com/community/en/conversations/optiplex-desktops/optiplex-small-form-factor-plus-7010-psu/66aa80d3a84db71337b5d11f?keyword=300w