I'm having this problem, too. Sometimes it seems like all the weight I've lost recently has come from my fingers and boobs, lol.
My wedding ring is tough (if not impossible) to resize because it has channel-set square-shaped stones, and the stones are all just pushed together, no metal pieces in between that the jeweler could just squish flatter to take up the space.
For now I've wrapped things around the back of the band (tape, Parafilm, etc) but I like the idea of getting a cheap little band that fits perfectly and wearing it "above" the ring on my finger to hold it on. It might have to be adjustable, or a childrens size, lol... my finger was a size 4.75 when I bought the ring, I can only imagine what it is now!