Why, AZ will be easy. Unless something changed, WhyNot, ND is a bit more difficult. I did the ride a ton of years ago and there was a store/shop a guy ran out of his house in WhyNot. I bought something - cold drink or?? - and asked for a receipt. Told the guy what I needed it for and he said I was crazier than he was.
Pretty sure I mailed the info in but don't know if I ever heard back or got a cert. I was working on upping my game some to ride the IBR so the ride cert. wasn't important. Since I got drawn and rode in 11, I'd guess somewhere in 10 was when I did the ride but that could be wrong. I used to have a place in Tucson so a long ride to Why was at least an hour or so.... ☺
Two locations in Why, AZ - one is the Why Not Travel Store right at the intersection of AZ-86 & AZ-85. The *other* location is about a mile to the east along AZ-86, the casino there just inside the reservation border (that's where I got mine...) has fuel available 24 hours.
Now, I didn't do the NC version, but the MS version. I thought the ride info said Seagrove was the alternate location for Whynot, NC. Apparently, in doing research (another one of those 'future rides for consideration', a couple of years ago), a lot of pottery places in the area, but might be difficult to time a ride to get a receipt at one of those locations.
@SteveAikens mentioned the long ride out to Why, AZ to begin a ride. I took some lessons from that and spent the night in Ajo, AZ for the first (and only) time I've done it. Think the next time, I might consider Gila Bend instead.
Why Not Grocery, MS is at 32.28473, -088.49065 this will save some time trying to find it. Use the casino in Arizona mentioned by kwthom you are more likely to get a receipt and it has better lighting with less coyotes at night.
Watch your speed on the Indian Community in Arizona. They enforce the speed limit and the Border Patrol gives the Indians a heads up if you are speeding.
Look at the receipt. In Why, AZ the pump did not provide a receipt. The Why Gas station was busy and the clerk gave me a random receipt that was off by 45 minutes. I bought a candy and received the correctly timed DBR. G.