well I dont know if you have $1 stores around there. Here they had cheap plastic window boxes the other day when I was in there. You might check.
If you are handy and can work with wood. Check at a local cabinet makers for scrap wood. Or look for pallets and use the wood to make your own. I am sure there are instructions on the web somewhere.
Good luck,
Kathy

A long wooden box would be easy to make from old pallet wood. Take off the slats, cut to length, and nail together.

The few affordable window boxes I've ever seen are way too shallow to be practical. My mother has some really nice white window boxes; deep, long, with nice detailing -- and very expensive. They still look good after three years on the house tho', so if you want to make the investment up front, there are some good ones out there.

Making them is quite simple, tho' and you can get exactly what you want with little effort.

I would check out freecycle -- if you do a search here you should find a link. The pine straw/needles sound like the perfect thing to ask for on freecycle!