What kind of toolbox are you looking for? A permanent over-the-bed box for a truck bed, or a removable storage bin?
I bought mine from Walmart, an injection molded weatherproof exterior box with a weatherproof masterlock. Removeable if I unbolt it from the bed, It's about 2' wide 40" long, and 20" tall, so it fits below the bedrails of the truck. It also has grooved feet to fit the grooves of the bed to prevent side to side sliding. I think I spent about $69.00 for it.
The one thing I'd be careful about if you choose a plastic box - I've heard a lot of people talk about them warping over time. Depending on the year-round temperatures where you live, this might not be a concern. I'm sure it's also dependent on the quality of the plastic used for the box, as well as the design of the box itself.