I've replaced my bottom with plexiglass lately for my chams old reptii breeze 18x18x36 enclosure that I now use for outside use. You can buy the glass and get it cut to the correct dimensions right there.
Remember thiugh that you need the piece of glass to be a little smaller than 18x18" so it'll fit.
You can use any piece of any inexpensive plastic.
Expanded PVC or Coroplast works great.
The Coroplast is cheap and you can cut it yourself to size. You can make a few pieces that way you just take out the dirty bottom and replace with a clean piece. Clean and sanitize the old piece and its ready to change out when needed.