I've always used au pair world or workaway (if you do go through workaway then offer to pay them though!) and found my own au pairs rather than going through an agency.
My advice is make your advert very very clear, explain the hours, level of spoken English you want, type of person you are looking for, etc. have a look at some other hosts adverts and get a general idea of what to say.
Then exchange plenty of emails/ messages and have a Skype interview as well. I Dont pick anyone who doesn't include a written personal message in their first contact or who would not suit us. The ones who are enthusiastic about my write up and explain how they would fit in well to family life get reply emails from me.
Really think about the type of person/personality that would suit your house (are you a loud energetic family or do you prefer more quiet and relaxed people, do they need to drive- think about age for insurance, do you want someone who has lived alone before).
Really question them on why they want to be an au pair - the best ones I've had have come because they want to improve their English (already at quite a good level but they wanted to become fluent) and genuinely love children. The one who Was a bit 'meh' just fancied a few months abroad because she couldn't get a job at home and had split up with her boyfriend....
My dc are still young so I prefer au pairs who have a sibling a lot younger than they are - that way they're already used to the occasional toddler tantrum (or the sight of a kid wearing knickers on their head and dancing around the house).
I guess an agency would do a bit of the filtering for you, but overall I've been amazed with my au pairs. It is quite time consuming doing it by yourself. The dc adored most of ours and we are still in touch.
If you're new to having an au pair I'd suggest advertising for someone for 3 months - that way you can see if it's something that would work for your home and your family and help you figure out if its