The last liner I bought (for my bog build) was from American Talapia (online). It was 20 x 12 and cost $139. The shipping was free. However, I don't think they sell EPDM. The liner I bought was HDRPE which is much lighter, but it's supposed to be just as good. It's also a bit stiffer, but in my opinion it wasn't too bad. It just takes some patience folding the corners. Think of wrapping a gift.
They say with HDRPE, In most cases, you don't need an underlayment. I guess that depends on your soil. I live in PA and my soil is clay and rocky. I was careful to remove any suspected rocks and just laid the liner right on the soil, no underlayment.
My pond is EPDM and my bog is HDRPE.
If you go with HDRPE, make sure it has the "R" indicating reinforced.