I assume you want to go from the RCA coaxial output from your source to the 3.5mm coaxial input of the Mojo 2.
Any RCA to
2 pole 3.5mm will work.
The Mojo end is a normal mono 3.5mm with the tip being the signal and the sleeve being the ground just like the RCA end.
The problem is many RCA to 3.5mm cables are designed to work the other way round meaning the 3.5mm is intended to plug into a 3.5mm output from a DAP and as such can be wired in one of various different arrangements to suit whatever the DAP manufacturer chooses. The Fiio set up for example uses only two poles from a four pole plug. These cables will usually have 3 or 4 pole 3.5mm at one end with RCA at the other end so avoid any that have that arrangement.
Any cable that has an RCA at one end and a
2 pole 3.5mm at the other will work as they have a standard wiring set up.
Another way is to get a little 3.5mm male to RCA female adapter that plugs onto the end of an RCA to RCA coaxial cable and into the Mojo at one end and the RCA output from your source device.
Generic photos for reference.