If you are looking to go old school, try Cheaper Than Dirt. They usually have original AR-15/M-16 slings for $10.00 or less! They also have other more contemporary options for pretty reasonable prices as well.
if you want to standard issue sling I have one you can have. I use the slong I took off my M-4 when I came home from Iraq. Adjustable and comfortable. Forgot the brand but it originally had a kagazine ouch on the butt stock end that I took off when I put it on my M-4.
I'm with fordnut.....Buttonsling.com is the way to go. The coast guard uses them and so did I when I was a deputy. They make the pin you install and then the sling just clips right on. I was part of the testing team with the CG when we tested all of the three point and single point slings and this one beat all the others hands down.
The sling you choose should fit the purpose you are using it for. I prefer a single point sling for when I am not carrying the rifle all day. I find it comfortable and it makes transition to my handgun very easy. In the past I have used the "Wilderness" single point sling. Very durable and is available with a quick release. I am currently using the Magpul MS2. I really like it. It is a few ounces heavier than the wilderness but you can configure it as both a single point or a two point sling. I've been using it daily for about 6 months now and the only wear and tear I can see is that the paint rubs off the hardware. No deal at all to me.