Overnight carer/nanny for teenage ASD kids

I've been a single mum who works full time for a long time now.  Up to now I've had help from my mum when my job takes me away overnight occasionally.  Unfortunately my mum is now unable to do this due to her own medical problems, so I'm trying to find solutions.

One thought was getting a nanny/carer in overnight on an ad hoc basis, just to get my teenage son to bed at a reasonable time and up for his school transport in the morning (he goes to a special school).  

Has anyone else done anything like this?  What's the best way of going about finding a suitable childminder who can do overnight stays and understands ASD? Are there particular organisations who offer this sort of service?

Thanks