SparkFun Solder-able Breadboard - Large and Spacers

I need to stack several SparkFun Solder-able Breadboard - Large, one top of each other. What size spacers do you recommend. The spacers I have now (M5 I think) are too wide to fit through the corner holes. Do you sell the hardware?

Thanks in advance.

https://www.sparkfun.com/products/11796