A 5lb CO2 tank weighs about 20ish pounds (the 5lb refers to the CO2 mass it holds), which is really not difficult to carry with a good backpack. I initially made my own using PVC tubing and some shoulder straps from a local army surplus store, but after about an hour it hurt terribly to carry. I now use <a href="http://www.nashvilleairsoft.com/index.php?page=shop.product_details&flypage=shop.flypage&product_id=848&category_id=89&manufacturer_id=0&option=com_virtuemart&Itemid=37">this backpack</a>, which is perfect for several reasons. First off, it has a great load distribution setup with very comfortable shoulder and waist straps. It also has four straps inside the main compartment which are perfect for holding the CO2 tank in place. The inside is just large enough for holding an upright CO2 tank and regulator, and the ports on the top which were intended for use as CamelBak lines make for perfect air hose placement. I can quite virtually wear it all day without getting very tired, and I'm far from a jock.
As FreeFall said, an air compressor just isn't going to cut it. You'd be best off buying a ~$50 5lb CO2 tank, ~$50 regulator, and the $60 backpack. I know thats a lot for an initial investment, but it is worth every penny.