What's the best way to get chisels?

I get like 1 chisel per 3-4 maps - is there a way to increase that rate somehow besides just trying to get higher-tier maps?

Kirac offers you a deal of 20chisel:8chaos for every mission opened, they are a frequent reward from Exarch altars and strongbox strategies drop them commonly as well.

Chisels are fairly rare with the current balancing though. I'd recommend looking into a cheap scarab setup first before investing into chisels as chisels are fairly expensive and add very little early on until you've invested into other aspects.

You don’t really need to use chisels until late red maps. I only started using them on T16. Much more important is to unlock atlas passives and get the +1 / +2 to maps. Also, make use of Kirac and buy maps from him to fill out your atlas quickly. Later on you’ll drop plenty of maps anyway, so don’t worry too much in the beginning.

Kingsmarch mappers give a decent amount

Kirac would be your best bet if you are not wanting to get them off trade. He will offer 20 more after each mission.

There are vendor recipe's for them. It is not likely to be time efficient to use it, but in certain cases, like league start or ruthless, you may find a use for it, and have your loot filter show accordingly.

Result -

1× Cartographer's Chisel

Input -

Gavel
with 20% quality
1× Any map

or

1 x 20% quality map

or

Maps that have combined quality equal or greater than 40%

If you are finding you always need more and are not able to get them from trade off the proceeds of the maps you are farming, it could be that you need to used them more efficiently.