I have both red and neutral-density gel - it's not really like a gel, in case you were wondering, but that's what it's called.
Got mine from a theatrical/film supplies store in Brisbane - I'm sure there'll be a similar store in every capital city. In case you can't find it closer:
Lee Filters at Production Shop
IIRC, I chose Primary Red (9.3% transmission) as my red filter - they provide a transmission spectrum for each product, so you can pick the filter that best meets your needs.
I made a cardboard frame for mine using thick card stock from Office Works.
Edit: I can't remember the exact sheet size (they cut it off the roll anyway) but one sheet was enough for 4 laptops.