Twinax cable is two small twisted wires inside a shielded cable. The proper wire to use has a characteristic impedance of around 100 ohms. There is also 72 ohm twinax available and it will work but not as efficiently due to power loss.
The cable distance between the reader and the antenna can be fairly long but there is power loss with longer lengths. The maximum feedline length can depend on the size of the antenna. It is possible to drive smaller loops at 100 meters but the distance is shorter for larger loops. The feedlines should be shorter with the Multiple Antenna Reader.
Feedlines should be placed away from power cables and other possible noise sources.
Contact an electrical supply company (one that sells to commercial electricians) and ask for pricing for Belden 9207 or equivalent