In my area they tend to give piles of them away for free when refurbishing tracks. The problem is that one has to take the whole lot which can sometimes be 1k+ ties.

I have also seen them do the same thing w/ wooden electric poles but those are more sought after.

Perhaps check w/ local utility/railroad crews that drive around and perform maintenance. My area is extremely White so our utility crews are generally accommodating and willing to help.

What's wrong with the berm?

This may be a niggerish idea but I am pretty sure they throw the old ties off to the side when they replace them, so you can just find some and pick them up. It might be niggerish because I am not really sure and maybe you would be stealing good ties by mistake.

You might check with a saw mill. They make four bark slices. You may be able to buy them cheap. Otherwise they are chipped up. My dad did that for the basement bar in my childhood home.