This is a local manufacturer, but a similar product may be found in your area.
I've used this type of mat in my square body trucks in the past.
Rubber animal stall mats.
4'x6'x.75". 100#'s. $65.00/Cdn. ...or under 50 bucks U.S.
4'x6'x.5". 75#'s. $55.00/Cdn. ...or just a bit more than $40.00 U.S..
I used the 3/4". I liked the extra weight in the back just for better, less bouncy ride.
Can't beat them for the price!
These ones have ribs on the bottom for drainage, & a textured top side that aids in grip.
http://www.summitrubber.ca/stall-mats.html
*Just say NO to plastic bedliners!*