This is another vote for Sleeve City Ultimate Outer 5.0 mil sleeves - a great product from a great company. I also recommend their Mobile Fidelity Master inner Sleeves.
Best Outer Sleeves for LPs
Is there a recommended type of plastic sleeve to protect valuable record covers from aging, yellowing, etc? I can buy them very inexpensively from the Princeton Record Exchange, but wondered whether they would have the desired effect. I've also seen poypropylene sleeves sold online at Sleeve City. Any recommendations? Thanks very much!