As a guy, I can't claim expertise in the jewelry department but will often purchase something of interest for the folks back home.
Here are two places where I've bought gifts and may be a possible sources for the materials you are seeking.
In town, try Tribal Pan Flute at 42 Hang Bac
in the Old Quarter. The store stocks a myriad
of unique, interesting gifts and jewelry from Viet Nam's 54 minority groups. I recall several beaded items and a a box or so of loose unique beads for individual purchase. The proprietress is exceptionally helpful, speaks excellent English, Mandarin and Vietnamese and may be able to assist with your inquiry.
Outside the city, try the the merchants or manufactures in Bat Trang Village for ceramic and related beaded items.
