I stopped at the main Cabela’s store in Sidney, NE to see if they had any isobutane mixture cans. Looked in the fuel section and found the butane cans I got there last year but no isobutane mix. I asked and she took be over to another section and they had the cans there. It’s an 80/20 mix in an orange can. $9 for the large cans. Brunton is the brand name. Looked on brunton.com and they did say it is a 80/20 mix. The Primus cans I had gotten before say 70/30, but I have to mail order those and pay a Hazmat fee on top of the can price. Cabelas is building more stores around the country so you might check them out if you cannot find the mixture in your area.
Thanks Jerry -
They opened a Cabela’s in Hartford a few years ago. I’ve visited twice and neither time did they have any Butane in stock. I asked and they said they didn’t carry it at this store. Odd - no one camps in New England ??? I’ll try again next time I’m in the area, but I’m not making a special trip 
Target used to stock Butane, but hasn’t for the last few years. Might be the hazmat thing.
I bought a two or three year supply straight Butane in the Asian style cans from King Butane. It was cheap and at the time they did not charge a hazamat fee on shipping. I’ve had no problems firing straight butane in my Shay, even in the winter. I do have to pre-warm the tank with a lamp before firing when it’s cold out (and in the basement storage area), but once the boiler has been lit for a few minutes, the closer proximity of the tank keeps it plenty warm for gas to form.
Jon,
My shay I made is fine also. Tank is right beside the boiler and stays warm. I made a big water bath around it also. We have a smaller Cabela’s in Kearney and they don’t have any of the fuel either. So guess it varies. Someone said they have found the mix cans at Walmarts back east, never seen any at stores here in Neb.