I buy all of my webbing & hardware from
Strapworks who offers a TON of options and great service.
A couple of things from someone who has made & repaired a bunch of bags, pouches, packs & straps in my life:
You will have to replace both halves (male & female) of the buckle because the likelihood a new male will lock properly with the old female is small, (just as in life 😉).
It appears the male half that is broken is attached to a slider on the shoulder strap pad via a short loop of webbing. If you have a sewing machine, it is a piece of cake to use a seam ripper to open the existing stitching on that loop of webbing, add the new buckle half and restitch it closed.
I'm not sure what goes on with the female half, but I'd venture to guess it is similarly attached and just as easy to open the seam and restitch closed. If you don't have a sewing machine, take Swattie's advice and visit the local dry cleaner or tailor.
A stitching awl will work too, but as someone who has used one of those a million times, It does a better job with heavy thread and wider stitches. I'd stick with DYI on your sewing machine or using a tailor.
Stay away from slip-on "no sew" options. They work OK on PALS webbing loops which are tight and flat, but I've had them slip off of the type of webbing loop you have on your broken buckle half.
Good luck!