Here is an interesting website that searches for stations selling B20 biodiesel. Minnesota is the worst state - now mandating 20% biodiesel at least in the warmer months. I try to never buy diesel in Minnesota. I also try to avoid Iowa and Indiana from my past reading but who knows.
The rest is a crapshoot. My local Freightliner dealer in Wisconsin tested the nearest gas station that was supposed to be selling B5 diesel max and it was considerably higher than 5%. Some of the stations sell premium diesel which is often labeled as not being biodiesel but I wouldn’t bet a lot of money on it.