You can find the nice automotive type at either Jeg's or Summit Racing. HR3 MaXout are nice.
You can use the smaller 1A:10B:C extinguishers from places like Lowe's or Home Depot if you just want.a good less expensive version. 3.3" verses 3" is OK. I do recommend finding one with a less intrusive (large) trigger.
I already posted this in the project thread, but I have to post here as well.
I received my Fire Extinguisher Mount/Release today!!! YAYY!!!!! I am very impressed with the build quality of this assembly!! Kuddos to you RD, and I will say that anyone sitting on the fence needs to get off it and buy one. You won't be disapointed!!!