I am looking for a paper suitable for printing stamp album pages. It should have the following properties:
(1) Acid free and does not yellow with age.
(2) Should be heavier than cheap copier paper but way below cardstock.
(3) Opaqueness: printing on next page should not show through on top page
(4) Should be smooth finish, but not slick and shiny.
(5) Should be white, not off-white.
Any recommendations, brands, samples? I need lots of this and a steady supply over time.

The first thing that you need to determine is whether you are printing with an aqueous inkjet printer or a laser/flat printer. Below are recommendations for both.

If you are using aqueous inkjet we recommend using one of our Moab Fine Art Inkjet papers. We suggest using Entrada Rag Bright in either 190gsm or 300gsm. Entrada is an archival acid- and lignin-free paper (no yellowing) with an expanded color gamut, natural contrast and high ink load. It is compatible with both dye and pigment inks ensuring that when used with archival inks, your prints will last for generations. With a smooth surface texture it checks off all of your requirements.

If you are running the paper through a laser or flat printer you do not need the aqueous inkjet coating. A 100% cotton, lignin-free paper will ensure longevity and prevent yellowing over time. We suggest using the following papers; Stonehenge White or Polar White 250gsm, Somerset Velvet Radiant White 250gsm or Saunders Waterford Hotpress High White 190gsm or 300gsm. All of these papers are archival, white and smooth but uncoated.