Dubizzle have them from time to time. I got mine from ebay in UK and had it shipped. 60aed for baby bjorn sling and 30aed for postege. Baby flea markets also worth a look.
Hi RT, I don't have a baby sling, but I do have a baby wrap that I'm trying to sell for AED100. It's in excellent condition and a great alternative to the sling. Personally, I think it's better, but I suppose that's all subjective anyway ;)
Here's a site about it for further info. If you are interested in the baby wrap route, then I'd recommend the sleepy wrap over the moby (another leading brand) due to the stretchy comfy material of the sleepy wrap. The material of the moby is a lot more rigid.
Let me know if you're interested.
http://www.sleepywrap.com/