I have one of these antennas (see Ruggedized Fixed Mast Antenna here). What cable should I be purchasing for it? RG213U? Is there any other I could use if I can't source this? Would RG213 be ok instead?
RG213 is the same as RG213U. The U means 'universal application' and is, more often than not, left off the description. Sometimes you see an 'A' before or after the 'U' and this means the cable has a stranded centre core. But if the 'A' is missed off the cable could still have a stranded centre core, such is the uselessness of these designations.
RG actually stands for Radio Guide, a US military designation, but it is also used to mean Radio Grade or, even, Regulated Government.
The numbers, RG58, 8, 213 etc are arbitary. RG58 is a 5mm cable, RG8X (or, sometimes M) is a 6mm cable and RG8 and RG213 are both 10mm cables. RG213 has better insulation than RG8 but is otherwise the same.
If you can get RG8 ('U' or otherwise!) it should be fine, RG213 is a bit more difficult to work with because of the 'bulletproof' outer jacket.