I always browse mods in the store.
I've created a criteria for what's worthwhile for me, but usually if one of my characters needs mods that I don't have, I'll spend a few weeks farming up mods in the mod challenges until I can roll some of those into worthwhile mods.
It's hard to fill gaps from the store because you don't know if they're going to roll the secondaries your particular character needs, i.e. you might find five purple/gold Potency + mods with speed in the store in the space of a month, and they could all roll non-speed stats easily enough. In that same amount of time you'll get more mods from the mod challenges, and thus a better shot at something useful.
May the RNG be with you, always.