I shop for Cypress chunks at AmazonMoosey, google it, its the first one that comes up.
I got this piece for Christmas:
40" Long by 8" Tall by 3" Deep
The people who run it are really nice. Just a word of warning though, if it says it needs to be waterlogged.. they mean it. I've let mine float since December and it still hasn't sunk. I've got it weighted down heavily right now. Also, take note of whether the piece is in stock or "on backorder". If its on back order then you know its gone.
They do go out and look for specific pieces as well, just let them know what size you need.