12v linear power supply

Zero Zone Ss2021. High quality torroidal with a large bank of capacitors, i.e. very stable and safely controlled. I use it to drive my Etherrregen and am very pleased with the performance. Spend what’s left of your $300 on a good fuse and power cord or a sterling silver DC cable. All on AliExpress

Automotive battery should be pretty good for that. Some of them can last before recharging driving your preamp for months.

Get simple low-current charger for it and you're all set 

Little more but Teddy Pardo is an excellent LPS. I have one with my RME-ADIFS and there is a noticeable difference.

PLEASE insist upon a power supply with a fuse on the output. That especially includes a power supply based on a battery.

Otherwise you may find that power supply to be very expensive indeed.

Swagman makes excellent linear PSU’s. I bought an 18VAC one, and it’s built well and works well. Hope that’s of help. Good luck!