Magnificent Hi Power-great find.
For magazines, go with Mec-Gar new production. They're at a decent price, from a reputable manufacturer (Mec-Gar is actually one of the OEM producers for FN and for the Hi Power when it was in production), and you won't have to worry about worn springs or spread feed lips.
BH Spring Solutions has them, including the 10 round 9mm ones:
https://bhspringsolutions.com/index....wer-magazines/
On a used gun there's a lot to be said for having it go through a total re-springing. It's not very expensive, and, other than on some of the trigger components, it's relatively easily done.
Again, check out BH Spring Solution for suggestions-and they have a superb collection of YouTube videos that'll walk you through the processes.
https://bhspringsolutions.com/
I've had a series of fairly extensive threads here on p-f you might want to read throught that might be helpful.
https://pistol-forum.com/showthread....sited-Part-One
The Hi Power is a surprisingly ergonomic, and very reliable pistol.
A website well worth perusing is the late Stephen Camp's,
https://hipowersandhandguns.com/ His widow is maintaining it.
His book, "The Shooter's Guide To The Browning Hi Power" I HIGHLY recommend-it's a superb user guide, and very well-written. Stephen's early passing shortly after his LEO retirement was a loss to the shooting and LEO communities.
https://hipowersandhandguns.com/RHP%20Ad%20Page.htm
You've got some great Hi Power/High Power (you'll understand the difference in nomenclature shortly...) expereince here on p-f, feel free to follow up with any questions.
Here's mine in it's current incarnation:
Best, Jon