Below is a list of a few websites that provide supplies and services related to preserving your movie poster collection. I'm hoping to add a few more, so please email me if you can suggest some more links.
Studio C - Probably the no.1 place in the USA for getting your posters linen backed and restored. I've been using them for almost two years now, and the standard of work has always been nothing but excellent. They offer a range of services, from basic linen backing to highly advanced restoration.
Movie Poster Arts Gallery UK - A UK based poster dealer that also provides a linen backing and framing service. I haven't used them yet but will do soon enough as I'm keen to try some UK linen backers. If you have any experience of them then pleaseemail me
Bags Unlimited - Stocks a stupidly large range of poly and mylar sleeves of every conceivable size as well as backing boards, boxes and just about everything else. Ideal for US collectors, but shipping is costly for people outside the US for larger items.
BCE Mylar - Another excellent supplier, that specialises in Mylar for all types of collectibles. Also sells backing boards and poster tubes and boxes. Again, ideal for US collectors, but pricey shipping-wise to the rest of the world.
Preservation Equipment Ltd. - A UK supplier which sells a wide range of conservation materials, but not specifically aimed at any particular type of collector. However they do sell a wide range of Mylar (polyester) sleeves as well as conservation mount board (plus much more that I don't have time to mention here), so they're ideal for the UK collector.
Conservation by Design - Another UK conservation supplier. Stocks Mylar sleeves of various sizes plus much more.