It’s worth remembering that most (in fact, nearly all, general purpose paper being manufactured today is “acid free,” and will pass the pH pen test. This is because the papermakers add calcium carbonate to the peper pulp to neutralize its acidity — not out of kindness to curators and archivists, but to extend the life of their machinery. Go into a Dollar Store and check their gift-wrap tissue, or an office supply store and check their copy paper, or a party supply store and check their rolls of table-covering paper. Odds are it will all be acid free, and a hellova lot less costly than anything from an archival supplier. That’s the good news.
The not so good news is that “acid free” paper, if made from wood pulp, is not “lignin free,” and lignin will decomose over time to turn the paper acidic. “Over time” is hard to define. It’s likely be be measured in years, and it’s doubtlessly dependent upon temperature, humidity, and how highly buffered the paper was to start with. Still for people and instututions that have to stretch their dollars, this is something to keep in mind.
Arm yourself fith a pH pen (the ones sold by art supply stores to scrapbookers (typically for a buck or less) are functionally identical to those sold by archival suppliers for $4.95 or more, plus shipping. Take one with you as you browse through various stores and check their paper products for acidity or the lack of it. You’ll be pleasantly surprised.