Guild stores or crafter.
Because there's 0 reason to use white glyphs. But they drop pretty often after the update, if you need them for deconstruction.
Dro-m'Athra Destroyer | Divayth Fyr's Coadjutor | Voice of Reason
Either look in a guild trader in mournhold/craglorn (best bet) or you buy them in the imperial city sewers from the general tel var merchant. it is 800 tel var stones for v16 if I remember.
The Tel Var Merchant in Imperial City only sells the V15/V16 potency runes ... and not the glyphs themselves. However, the price was cut in half recently to 400 tel var per V16 potency rune. Just get a guildie or friend to craft the glyph if your own potency improvement passive isn't maxed at 10.