Hi ma stands for mili amp, so 500ma is equal to 1/2 AMP so you buy a 1000 ma power supply which would be capable of 1 AMP which would be good for what your saying assuming your load does not exceed 1 amp. Do you understand.
1000 mili amp = 1 Amp that is the relationship
500 mili amp would = 1/2 amp
750 mili amp would = 3/4 of an amp
1000 mili amp would = 1 amp
Hope this helps
Alan