Ladybirds

I have quite a bad greenfly problem in my garden this year. My silver birch was absolutely covered! I bought some spray for it which has cured the problem, but it's nasty stuff and I don't want to keep using it everywhere.
So, I'm looking into buying/encouraging ladybirds.
A few Qs-
Is the attractant you can buy worth it?
What's the best type of house?
Where's the cheapest place to buy larvae?
Can I safely introduce them having previously used the pesticide?
I was just checking for greenfly on my clematis and there was a ladybird there with a little clutch of ladybird eggs on the fence behind, so it looks like I'm might be off to a good start