I've used this 3M distributor in the past:
R.S. Hughes
1100 Corporation Parkway
Raleigh, NC 27610-1300
(919) 231-3800
R.S. Hughes is on the east side of Raleigh, off of East New Bern Ave. / old Rt. 64 (outside the beltline). There's a traffic light at the intersection and a Circle K on the near right corner. They should have such a common item (as the P100 filters) in stock as they carry a wide range of 3M items (respirators, finishing products, adhesives,etc.). However, I'd call ahead just to make sure. Also check on the 'to the public' sales, they used to tell me 'if you have money, you're a customer'. I believe their prices will be better than Grainger's.
I think you've made a great choice in particulate filtration. Nothing beats the 100 rating filters. When I ran a sign shop I asked the 3M tech line about the best filter for lead paint. They suggested the Organic Vapor filters with the P100 pink pancakes on top (they had to be mounted with adapters). The tech told me that the P100 filter was so fine that the lead particles probably wouldn't even make it past that level of protection (the lead based molecules being so large). The Organic Vapor cartridges for the contact cement is spot on but obviously you don't want to use them for particulate filtration as that would clog them with dust.
BTW - here's a clarification of the 'N, R and P' codes found on masks (if you weren't aware):
N for Not resistant to oil,
R for Resistant to oil
P for oil Proof
Dean