I've been in your situation in the past. A replacement piece of
tempered glass for the patio table costs more than the table.
Table sizes are somewhat standard. I found a neighbor selling
a patio table in a garage sell for $20. The table was fairly beat
up, but the glass top was ok. I bought the table, inserted the
glass in my table, and tossed his beat up frame into the trash.
I also learned to toss a sandbag on top of the patio table when
the weathermen are predicting strong winds. The umbrella must
also be folded up and, better yet, removed before a storm. I
estimate that there were over 3000 tiny pieces of tempered
glass on the patio after the windstorm tipped over our patio table.
(Yes, I am extremely good at estimating). A concrete filled
umbrella anchor also helps.
You are going to have a difficult time finding an inexpensive replacement
table top that you will be happy with. You could check around on
plexiglass suppliers in your area, but you will probably have to
purchase a 4'x8' sheet of 3/16" plexiglass (acrylic sheet) and the cost
will be over $100. You could buy a new table for that price. And
with the new table, you will have a top which won't scratch easily.
You could make a plywood top, but it will need bracing since even
an oversized 1/4" (4/16") sheet of plywood won't be very strong. And
it is going to look pretty crappy. And it won't look as nice. And
it will need maintenance. And you won't be able to check out the
legs of your female guests.
If you are extremely luck, you may spot something sitting out on
trash day which could be cut and improvised for a table top. It will
be free and the regulars on this newsgroup know that I'm a big
advocate of putting trash to use.
Do NOT use standard glass for a patio table top. Do NOT consider
cutting down a piece of tempered glass to fit your table - glass can
only be cut BEFORE it is tempered. (Been there; done that.)
Do not consider paying somebody to temper a piece of glass after
you have cut it to size - that is way too expensive.