Anyone know a source for A4 cardstock in the states?

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Here is a source for cardstock that can be trimmed to A4 size.
Officeworld.com store

In the first search box input Pacon Tagboard.
There are manilla and white available.

Check out the medium weight 12" x 9" tagboard. It is around the same as 65lb (cover) cardstock or 177gsm.
My caliper measures it as around 0.21mm / 0.008"

The heavy weight stuff is around 252gsm or 93 lb cover and 140 lb index. My caliper measures it as around 0.27mm / 0.0105"

The 12" x 9" tagboard can be trimmed to A4 size. The trimmed strips can be used as suggested in this thread and other threads relating to this topic.

I have ordered from Officeworld.com and have received prompt service. Most orders over $49 USD ship free. Shipping for a tagboard order under $49 USD is very resonable.