I recently had to start wearing varifocals which are bigger and heavier than my old glasses. Positioning is critical for the focus to work so the arms are set to grip my head much tighter, uncomfortable at first but I have got used to it. The added bonus is that they don't slip down my nose.
I guess the point I am trying to make is that you may not need to buy anything, just a bit of frame adjustment required which most opticians will do for free.