If you are wanting childcare for a newborn, I feel you should aim for a nanny, not a mothers help - as I feel a mothers help is more someone to help around the home with household duties, rather than actual childcare.
Ideally I suspect you want someone who is a master of all trades, so can help you with whatever needs doing at the time - catching up on laundry, cleaning, entertaining 3.5 year old, or encouraging newborn to sleep.
Try advertising locally, newsagents shop window for example. As you want someone live-out, the nearer they live to you the better. If you can be flexible in the hours you require, then with luck you can find someone already working in another job (or jobs) and who wants to add another small job to the ones they are already doing.
To be able to advertise though, you really need to work out what it is you ideally want, so that you can then advertise for say someone to do 10 hours per week, spread over say 3 days, but flexible on days and hours.
Alternatively, commit to a specific number of hours - or to having help only on specific days, as if someone works 3 days per week, they may be looking to fill 1 or 2 days per week, but they may need to be full days so they can pay their bills.