Kevin Lee tools has basic diamond and French irons that are really affordable ($30 for a set of diamond pricking irons - 1,2,4 and 6 teeth) and he does have 5 spi. According to the website, they're low cost, no frills so people can try out new tools without breaking the bank. I got some of the French pricking irons from him a while ago, and they are great tools at a great price. Teeth are polished, too, so they don't stick. They're made in and ship from China, which I know you aren't crazy about, but they're a great deal if you don't mind the wait...and China.
My diamond chisels are from Weaver, and the biggest one in the set sticks pretty bad, especially in thick leather. They're not bad tools, but they're also not made in the USA, I don't believe. Good luck with the search!
As far as using them goes, getting them to come out evenly on the other side takes a little practice. My lines still wander more than I'd like Also, I find it's harder to keep the backside line from wondering any time the shape of your piece prevents you from having the stitch line lay flat. Also, with really thick leather the chisels won't punch all the way through. Prongs are too short. You can drive them as deep as possible and follow up with an awl, but taking them all the way down can mark the face of your project, and if the teeth aren't polished you spend a lot of time trying to get those things free of the leather.