You can find NAPTHA at Lowes or HD in the paint section, in quarts and gallons.
I used to buy Coleman fuel as naptha, but the last batch has a way different smell and not as agreeable to me. Good old fashioned lighter fluid is naptha. Compare the smell to be sure.
Isopropyl Alcohol can be bought readily at drugstores in 2 formulations. The most common is 70% (30% water) and 91%. I find the 91% at CVS. I also order anhydrous ( sp?)100% for special shellac preparations but it's 3-4 times as expensive as the 70%. I’m not sure as to whether or not the 70% would be better for decarburization, since water sucked into the engine is how we did it in the 60’s.
Looking at the formula again you have a range from low side 85%- 115% and if you take the mid range number, you get 95%, so I conclude it just isn’t that critical. It’s roughly about 50% pale oil, 30% naptha , and 20% Isopropyl. I’m thinking (always a problem) ½ gallon pale oil, 1 qt 91% isopropyl, a little over a quart of naptha to make a gallon.