I know what you mean! I'm considering using green metal fencing to make mini cages. I've been looking for plastic cages that will snap onto the rim of my 8 inch plastic pots. A place in Great Britain sells them for Achimenes with pots included (too small). The best solution I've come up with is to insert 3 stakes around the perimeter of the pot and then loop those Velcro plant ties around the Begonia stems from multiple directions. Eventually the rootball fills in to support the stakes.

I have my pots sitting on/in much larger pots filled with clay soil so they dont fall over and it holds up stakes nicely. The begoinias grow a few roots into the clay and dont wobble anymore.