Mdf is mdf more or less. I get mine in full sheets 4’x8’ from rugby architectural building products, the only caveat is that I have to transport it and cut it to size. It’s very inexpensive though, works out to about $0.50 per gf sized sheet.
Rugby has a lot of locations, maybe one near you? Rugbyabp.com.
Oh and if you want it masked that’s up to you to do too.
For more info #1a and #2 are probably of interest to you: