Well,not exactly the east coast - but from SoCal, the midwest seems like the east coast, lol. It looks like Ohio and Wisconsin seem to mfg them – I find it hard to believe only midwest clients want them.
I used to use Summit Rubber, but it looks like they don’t offer them anymore.
I checked at my local feed store and they said they used to carry them, but no one wanted them because they were more expensive.
Tractor Supply looks like they will ship to your local store for free, but I didn’t read the fine print. Their interlocking mats are at a stiff premium – a regular 4x6 mat is ~$60, but the interlocking ones are $163 each!!
I have a feeling I am going to have to use straight-edge mats…