where can I get paid initial installations for my new mobile app in USA and UK?

Some mobile apps have this problem of empty bar - in theory the service is cool, but as there are not many users, it is not so attractive. There are not so many users, so others won't try and stay.

I have heard that there are companies that can arrange you some paid first users installations in selected cities, you can also select some of those users characteristics (ie. females, runners). Anybody have used such a service and could give me some directions to start with? Thank you!

Those kinds of services are almost always trouble. I would personally stay away. Find the smallest demographic and number of people needed to get critical mass in an area, and promote it like crazy. If that's your own town or city, then get your friends using it. If you're really after female runners, then talk to the local running store and running clubs and sell them on how great your product is. Tell them you'll donate money to their organization if they'll sign up, try it and give you feedback. I'd be very wary of the services you're describing.

Sounds like a scam. At best, they'll use your money to create fake sign-ups, then cancel. At worse, they'll use stolen credit card details.

As Jeremiah says, find a niche where your product can make a difference and be the "go to guy", who knows all the answers. Once you're established, expand into neighbouring / complimentary niches and keep going. Hopefully, you'll have some happy users who can evangelise for you too, which will make things easier as you go along.