Regulators made for use with a gas are basically all alike mechanically. They may be one stage, where the pressure is dropped in one step from inlet to the adjustable set outlet pressure. Or, they may be two stage, where the pressure is first dropped to a fixed intermediate pressure, then to the variable outlet pressure. The latter works far better for our purposes.
Regulators need an inlet fitting that will mate with the industry standard gas tank fitting for the type of gas it is to be used with. For CO2 that is called a CGA320 fitting. This fitting screws into the inlet port of the regulator.
Regulators are designed to work with a fixed range of outlet pressures, which can be 100-500 psi for example, or, what we want, which is 0 to 30 psi. That range isn't adjustable once the regulator leaves the factory.
Regulators made for industrial use, or for laboratory use are made to meet the demands of those users, which are far higher than the demands of aquarium keepers or even beer users.
With those facts in hand, look for:
A 2 stage regulator, identifiable by having a big bulge at the back of the regulator for the first stage.
One with the outlet adjustable up to no more than 50-60 psi, preferably 30 psi.
If it is a used one, look for an indication that it was previously used in some kind of laboratory.
Pay no attention to what gas it was used for - you can replace the inlet fitting if needed at little cost.
Make sure the maximum inlet pressure is above 1000 psi and below 5000 psi, so you can easily tell if the inlet pressure is 800 psi or 400 psi.
We aren't supposed to post links to Ebay listings anymore, but I enjoy looking for good regulator buys, so I have been known to PM someone with ones I find.
It is very easy to assemble one of these if you have moderate ability to use a wrench.