It's bond type paper like is used for magazines. Worthless for inkjet printing. I find that almost any decent paper looks much the same when put under the plastic sleeve. Go with any good quality inkjet paper and you'll be happy. If you want something shiny, brochure paper is good. (HP sells one).
Personally I use cheap 28# copy paper with high-res printer settings and the results look fine when it's inside the sleeve.