Hello, There's numerous places to buy Sleeves. One, is Sleeve Town (Or is it Sleeve City)
Usually places like Elusive Disc, Needle Doctor, Garage A Records sell them.
I have always bought my Poly Lined Inner, and Outer Sleeves from MusicDirect. www.amusicdirect.com
While the Mobile Fidelity Sleeves are really nice, they do also cost a bundle, and generally are sold in quantities of 50.
The Inner Sleeves I have usually bought from MusicDirect are a high quality Poly Lined Paper Sleeve, are very well made, are quite a bit less money than the Mo-Fi's, and can be gotten in lots of 100. (They may still break lots of these to sell lots of 50 also, you'd have to inquire)
As for Outer Sleeves, yes, they are a wise investment, and if you only use Inner Sleeves, IMO you are only doing half the job of protecting your LPs from Dust, and wear, and tear.
The very best Outer Sleeves I have ever bought, and used, were the 6-mil Resealable Japanese Sleeves that MusicDirect sells.
These are not a "stretchy" plastic, where when you go to open the Flap, the Sleeve itself wishes to stretch, and tear. I've had these, and didn't like them, and returned them once with an in store purchase.
They are just large enough to hold some "thinner" double LPs, and will contain any of the single Mo-Fi releases, but will not hold any of the occasional triples you might have. Hope this helps you,
Bes Nivera is the guy to speak with at MusicDirect, ask for him. He'll know what you want, as they carry a plethora of Inner, and Outer Sleeves in various price ranges. Mark