You can go to
OfficeWorld.com - Office Supplies & Business Products! then input Pacon Tagboard in the first search box. The tagboard comes in 100 sheet packs.
Check out the medium weight 12 x 9 tagboard. It is around the same as 65lb cover/97lb index cardstock or 177gsm. $5.19/pack.
My caliper measures it as around 0.21mm / 0.008"
The heavy weight stuff is around 93 lb cover/140 lb index or 252gsm. $6.29/pack.
My caliper measures it as around 0.27mm / 0.0105"
I have ordered from Officeworld.com and have received prompt service. Orders of $49 US or more get free shipping, otherwise is it around $5.95 US for shipping.
There are white and manila available so make sure if you order that you pick the correct one.