Make a few phone calls to local hardware stores and/or glass and window shops. Most will cut the acrylic in whatever size you want. Be advised that you will generally not be paying for only 5x5 piece, but for the size piece that the store used to cut the little one from. Usually that would be an 8x10 . . .sooooooo if you planned ahead to figure out how many rulers you could get from that size, you may have to pay a slight cutting fee (our store charges $1 per cut) or not (we usually don't charge for any cuts if you buy the acrylic from us).
By the way, what you want is plexiglass. It is more expensive than real glass (go figure). You can probably get by with 1/10 inch thick, which is the thinnest we carry at our hardware. The thicker the plexiglass, the more expensive.
Plexiglass isn't the easiest thing in the world to cut; hardly worth the investment in the cutting tool when you can usually get it cut to size for no charge.