Welcome.
I second the Esstac recommendation. By far the superior choice.
As far as slings, I think that first you have to decide in what position you want to carry your shotgun while slung.
Some want it across the chest and "ready" to come up to cheek weld instantly. The "wrong" sling and mounts will have gravity flip your shotgun upside down which is no bueno.
Some want to carry over the shoulder and pointing up or down similar to how the military marches with slung rifles, but they are slower to bring to the ready.
Hunters slings are a dime a dozen at places like Academy and Wal-Mart and Cabela's. An entry level sling will run twenty bucks or less. A Magpul or Blue Force Gear will run over fifty. Don't waste your money on some sling that is beyond what you intend it for. But if you want a fighting shotgun, then you will have to spend $50-$85-$100 to get that. And please, never ever EVER fall for a 3-point sling. Or for that matter, even a single-point sling.
If you want to throw your gun over your head and have it lay properly across your chest, then I am the guy to ask.
If you want to carry like duck hunters going to a duck blind, then Wal-Mart can hook you up pronto and cheap.