Greetings mommatnt, welcome to the Forum !
As Thatguy says, a Hardware store, a regular Glass Company, one of the Big Box Stores, there's lots of places where they will cut you a piece of Glass.
Just take the measurements, and go get a piece cut to size.
If you can get the broken pieces of glass out of the frame, and lay them out on a table, measure that, cause the Glass is always a little smaller than the hole it went in. You don't need to reassemble every piece, just enough to get two sides put together, where you can measure it.
AND, you don't need to cut the hole bigger, i think someone is sorta "pulling your leg a little" !
Have a good Day !
S.W.