RG-58 isn't that bad for reasonable lengths. Even at higher frequencies. I've got about 50 or so mobile 800MHz radios at work, and they are all using NMO mounts with RG-58 of various lengths.
If you just want longer cable, you can get mag mounts with up to 125 feet of cable:
Laird TE Connectivity GBR8 (Cable up to 125 Feet) Extra Length Cable - Manufacturer: TE Connectivity; Description: Magnetic Mount For use with NMO style
theantennafarm.com
Or, if something like 17 feet will reach, you can get mounts with higher grade coax:
Here's a mount with 17 feet of LMR-195:
EM Wave EM-MAG-195 NMO Magnet Mounts No Connector - Manufacturer: EM Wave, Inc; Product Description: Magnet Mount Base, NMO, Black w/17' Low Loss 195 cable
theantennafarm.com
Unless you need a really long cable, or you are running way up in the GHz range, RG-58 will do just fine.