I bought some to use as door panels in my 4x4. The sheet was 4' x 10', 1/8" thick. If I remember correctly, it was about $80 from a metal supply company. Look in your yellow pages, do not buy from a home center or tractor supply - the prices are about 10X higher.
You can easily cut aluminum in straight lines with a carbide tipped circular saw. VERY IMPORTANT to wear long sleeves and at least safety glasses. A full face shield would be better. The chips are hot and sharp and they will go everywhere. If you need to cut a curve with a jig saw, use a coarse blade and change it as soon as it stops cutting - the Al gets "sticky" on steel and clogs the teeth.