Reddit reviews Online Plastic Supply Acrylic Plexiglass Sheet 1/4" x 24" x 36" - Clear

For the plexiglass, I think you can get something like this:

http://www.homedepot.com/p/OPTIX-24-in-x-48-in-x-093-in-Acrylic-Sheet-MC-13/202038048

or this:

https://www.amazon.com/Acrylic-Plexiglass-Plastic-Sheet-24/dp/B00C13ZSBA

Honestly, I just walked into a Home Depot and asked for a big sheet of plexiglass. It was about $25. Someone above suggested that Hobby Lobby may be cheaper.

Here's the battlemat:

https://www.amazon.com/Chessex-Role-Playing-Play-Mat/dp/B0015IQO2O

Grab some whiteboard markers, and the whole thing will run you about $50. But honestly, I've used this same setup for 4 years and it's been great.

A few thoughts:

• The plexiglass doubles as a great protective surface for whatever table you're playing on. We've been saved from many spills because of it!
  
• The plexiglass is a little heavy and cumbersome. My group meets at a different place every week, and it's a little annoying having to carry around everywhere. It's great if you have a dedicated gaming room, though.
  
• The plexiglass can get dirty, but I never notice while playing. Just wipe it down with windex every now and then.
  
• Get colored markers! It's so nice being able to track hp in red ink right next to minis as they move around the map, and my players love drawing out possible tactics/formations, etc.
  

You can cut acrylic sheets easily with a circular saw, band saw, table saw, etc. The edges do not have to be perfect since it will be hidden in the frame.

http://www.amazon.com/Acrylic-Plexiglass-Plastic-Sheet-24/dp/B00C13ZSBA/ref=sr_1_1?ie=UTF8&qid=1462206658&sr=8-1&keywords=acrylic+24x36

Edit: depending on your frame, 1/4" might be too thick. I believe it is common to use 1/8" for framing.