Preserving and Protecting Stamps

I agree that acid free glassine interleaving is probably the best option. My double sided stock book came with interleaving so I haven't had the need to retrofit it. Is your stock book hardbound or loose leaf? There are several online suppliers in the U.S. that sell glassine interleaving for use in 2-post or 3-ring stock books (Amos, Mystic, iHobb, to name a few).

Then any of the three vendors I mentioned will have what you need -- a package of 100 sheets costs between $10 and $13. You can also find the interleaves being offered on auction sites like ebay and Delcampe.

GLASSINE

In the interest of better Philatelic husbandry,   here are various sources of information.
Members can digest, and make their own opinions of how best to store long term,
and be aware of some of the negatives, and positives of Glassine.


Glassine is manufactured by a process called calendering; after pulping and drying,
the sheets are pressed through rollers such that the paper fibers flatten out facing in the
same direction. In order to maximize the smoothness and thinness of the paper,
glassine must go through this process several times, and so is referred to as
supercalendered.

Usage
Glassine has a number of practical uses. It is frequently employed as an interleaving
paper in bookbinding, especially for illustrations; the paper can be manufactured with
a neutral pH, and can prevent damage from spilling, exposure, or rubbing. Glassine
tape has also been used in book repair. In chemistry, glassine is used as an
inexpensive weighing paper. It is also used in foodservice as a barrier between strips
of products (e.g., meat, baked goods); glassine is resistant to grease and facilitates
separation of individual foodstuffs.
Philatelists use glassine envelopes to store stamps, and stamp hinges are sometimes
made of glassine.
Amateur insect collectors sometimes use glassine envelopes to store specimens
temporarily in the field before they are mounted in a collection. Entomologists
collecting for research purposes may likewise use glassine envelopes to store whole
specimens in the field. However, if the research involves DNA or other tissue analysis
the relevant parts of the body (e.g. abdomen or testes) will typically be placed
immediately into a preservative such as ethanol. When this is done for Lepidoptera
(moths and butterflies) the wings will often be separated and stored indefinitely in
glassine envelopes in case the wing patterning or venation is required later to confirm
the identity of a specimen.

Glassine envelopes have also been employed for carrying drugs such as cocaine and
heroin.
In the mid-20th century, potato chips were sometimes packaged in glassine bags.
Glassine is sometimes used in the packaging of firecrackers