I have bought pure stevia and sucralose from Bulk Powders a while back. Not experimented much with the stevia yet but the sucralose tastes very sweet. I made a 20% solution of sucralose in AVG. 10-15 drops of this in 10ml of juice is plenty. Also find it sort of adds a powdery taste which i quite like.
Stevia will dissolve in PG (not tried VG) but needs warming every time to redissolve it.