I have been buying 12"X12" Basswood from Amazon. $37 for a 20 pack. (link below) This particular supplier has square corners. other suppliers have rounded corners so you loose a little. The quality has been great. I have also bought 4x8 sheets of 5mm poplar from Lowes for $27 but they don't always have it in stock.

https://www.amazon.com/dp/B09TR63TSZ?ref=ppx\_yo2ov\_dt\_b\_product\_details&th=1