You may have to do a bit of searching on their website, but Light Impressions has all kinds of storage envelopes for archiving, as well as presentation. I don't know what their selection for LP's might be, but they are worth trying. You should be aware that any printed media which was printed by offset press, as album covers were, will be prone to fading if exposed to extended periods of sunlight or intense light. It is the UV component of the light that fades the inks. Light Impresssions, as well as other archival suppliers, do sell UV filtering glass and plexiglass for framing. Using this kind of glass filters out a large amount of the UV light and will extend the life of any color media. I would still recommend you keep it out of direct sunlight (an intense source of UV).
Marco
Marco