Before I give any advice, it would probably be a good idea to list what you're looking for in a bag/sleeve, what your price range is, and what you'll use it for. You'll get much better answers that way.
However, I will say that you might be better off buying a decent bag for your computer, rather than looking for the cheapest one. You've spent over $1000 on your computer. Do you really want to be carrying it around in the cheapest bag you can find? I'd suggest buying a bag that will protect your computer extremely well, and last for a long time.
The two brands that I have a great amount of trust in are Brenthaven and Booq. Both of those brands' products cost a bit more than the average bag, but you'll definitely get what you pay for. I own and have owned products from both brands, and haven't had any complaints whatsoever.
I personally have a
Brenthaven Fusion MB messenger bag, which I love. It's got an incredible amount of protection for my MacBook, and looks good too. If you look around on the net, you can find one for just over $60
here.
Again, I feel that you definitely get what you pay for, and you're much better off getting a quality product right away, rather than regretting it in the future. Granted, I understand that budgets do exist (unfortunately), but when you're getting a product that will protect a large investment, it sometimes pays to spend a little bit more initially.