I use the Merc ones. They are bullet proof and work. As the should for the price.
My brother had made one for his jet boat from regulator from Rural King, farm implement store) that are used on sprayers. I think he said he paid under $10 for it. Been a few yrs.
Or search for a Watts relief valve. They have 2 sizes I know of 1/2 and 3/4". I tried the 1/2" purchased thru Teague. (not knowing it was simply an over the counter item) It was too restrictive and not low enough pressure adjusting, and proved useless.
Make sure you are controlling pressure, not flow. I see these 2 conditions confused all the time here.