quick fuel carbs sells them
also Merc also Volvo penta and also Sierra aftermarket marine parts. ebay has them also.

I bought a bunch when Mallory Marine was in business as well. They are smaller but worked just as good. My cost was like 5 bucks at that time frame.

Quick fuel was around 18 buck if I remember correctly.

Fuel testers has been around for a while.

I use mine all the time. In my area all gas that I see in boats are 9 to 10 % / E10 never seen anything over that percentage as people claim else where.