I think most all panel mfg provide the side-by-side space saving breakers.
You may even find a generic that will fit your panel.. You need to go to an
electrical supplier that handles all the breakers.

You might want to consider upgrading to a 200 amp service anyway.. That is
pretty much the standard for a residence today, even if you don't use elect
stove, etc.

What it will cost to upgrade depends on the going rate in your area. The
panel will be just a small cost in this change over.. I purchased a medium
size sub-panel that holds 16 breakers for about $80.

A sub panel isn't going to help you since your really need to bring in more
amps (200 amp).

What ever, it will add future value and piece of mind with any other
additional system expansion.

Steve