For replacing broken playfield glass you have several options.
Normally machines come with tempered glass. When this breaks you get
zillions of itty-bitty pieces that fall into every hole and switch, and
create problems for a long-time until every piece is found. In addition,
tempered glass has been known to spontaneously shatter. If safety is
critical for you then you really have no other reasonable alternative. If
you acquire the glass at a local glass shop it will cost $30-45 although it
has been known to be available sometimes from special sources for $10.
If safety is not a concern you may want to use regular plate glass. When
regular plate glass breaks you end up with just a few, large, sharp pieces,
which are easily cleaned up, but are dangerous. Normal play (and
pounding) for pinballs will not break plate glass. If you acquire the glass
at a local glass shop it will cost $15-30. Be sure to get the edges
'sealed' so you won't get your hands cut when handling the glass.
In either case get clear 3/16" glass 21"x43". This is the standard size.
If you have a widebody pinball you will need to measure before ordering.