Most paint manufacturers offer single stage urethanes or polyurethanes. That is what Matthews is. Matthews has the advantage of offering an online formula cross-referencing system that will give you paint formulas to match a huge number of colors from most major paint makers, as well as manufacturers' colors of other things, such as vinyl, Dryvit, prepainted aluminum, vehicle fleet colors, federal government colors, Pantone colors, Rowmark plastic colors, standard Gemini colors...the list is seemingly endless. Or send them a sample and they will match it. This matching service is free.
You just need a mixing station.
Before we had a Matthews station, we simply picked an automotive color and had the local Dupont jobber mix it. Not convenient, but it worked.
We have also used the Sherwin Williams line of polyurethanes. Again, not convenient, and most Sherwin Williams retailers do not carry them. Most of the Sherwin Williams stores are not even familiar with these paints.
The Grip Gard system is good.
Matthews, however, leads the way in both product offerings and customer support. Their training manual and free 2-day seminars have no equal in the industry. And the focus is on sign paint, not autobody or house paint.
If your volume is smaller, there are mixing stations available for quart size cans.
A working relationship with another sign shop that has a mixing station would be another alternative to having your own.
A mixing station is not cheap, true. But it is an important investment for a commercial sign shop to be competitive, in my opinion.
Polyurethane is the best choice for painting things like channel letters. No other readily available paint will offer the same durability.
Brad in Kansas City