There is ever-ready.
Permanently accessible, available, or prepared: 'Roxie’s ever-ready smile'
There is handy also which is more common.
Readily accessible: found a handy spot for the hammer.
Note: I gave my answers based on the exact definition only. There are different words and phrases that can be used based on the context. If it is related to information technology services, using "available 24/7" would be better.