I bought my left from Chevy Duty a couple of years back. Truck freight is expensive! I paid right at $400 for mine by the time that I paid the freight which was $150 plus the $250 that the bedside cost back then. They ship it to you on a pallet via 18 wheeler, or at least that's how mine showed up, on a Yellow Freight truck. No UPS here.
The quality from Chevy Duty isn't that bad but I must warn you that you have to make it fit to line up with the tailgate to make it look right. Lots of tweaking and adjusting. And you will have dings and dents that will need repair. I don't think that it matters where you get it from, it will be the same story.
I bought a rust free original GM bedside off of an 87 for mine at the swap meet here in Springfield, Mo. this past weekend. It will be going on my truck as soon as I get a chance to install it.
As far as other sources, I shopped around before I bought mine, but Chevy Duty beat every one else by the time you figure up the price of the bedside and shipping cost. Of course I live just 3 hours from K.C. so that helped me.
Brothers has them but they are $349.95 plus truck freight.Classic Industries lists both aftermarket and original GM in their catalog, but the original GM is well over $1200 per side plus truck freight!
You might also want to check Cheyenne Pickup Parts.
Goodmark doesn't sell bedsides for the 73-87s as far as I know. You might also want to check with the vendors on this site as well.
The thing that really kills you on these things is the truck freight. I would check locally and see if you could find one that way first, then if not, decide which one that you would want to order from. It seems they are all different on the price of the part and truck freight.
Oh yeah, I was told when I ordered mine that if I had needed both sides it would have been one shipping cost for both. It would save you a little bit of money if you can still do it that way, Ask them and see what they tell you, it sure won't hurt. ...Joe