You can post online or local ads, but if you prefer to work with someone in your country, you can specify it. It's also very different to "partner" with someone or to hire that person as a contractor; you cannot either expect the same kind of loyalty from a contractor you will get from a partner.
There are a few things you need to consider and that can give you some clues on where you should look for freelancing partners:
You need to make sure they sign a Non-Disclosure Agreement (NDA)
with you before the "interview." Trust is very important but the law
is what will protect you in these cases. That's why you also benefit from hiring someone in the same country as you. It's also very important to
add clauses that forbid the contractor or candidate to contact your
clients. I suggest to be transparent with your clients if you use a
contractor but this is your personal decision.
You might want to hire someone first and then offer a partnership agreement after testing her/his skills. This gives you more
control on the information you're expected to share with them and
what you prefer to keep for yourself. This is a good investment for you.
If possible, find a partner that is a full-time freelancer as well.
The reason is that many freelancers do it as a side job and they
will always prioritize their full-time job first. It's important to
find someone who has the same goals as you and understands your
priorities. They're also usually more reliable than part-time workers.
If you have a potential candidate in mind, go have a look at their
social media profiles and do a good search online with any of the
email addresses you have that belongs to this person, pseudonym,
interests, etc. This way you'll learn a lot about the candidate
his/her level of maturity, understanding of privacy and
professionalism. That will also help to add important details in
your contract that could save you a lot of troubles later (eg.
people posting drunk pictures or saying negative comments about
their clients online or people with a "Steam" gamer profile with 30hr a week of gaming!)
Ask for a Skype interviews, phone conversations and meetings. Lot of
things can be read through body language and the voice tone (eg.
If you ever, EVER have a little red light or weird feeling about a
candidate, listen to your guts instinct and try to not compromise.
If you aren't sure of something, do your investigation, don't feel
uncomfortable to ask questions For example, if the candidate says
things like "My wife will help if I can't do all the work..." This
is a big NO; you don't want to be stuck with a third party that
isn't fully part of your NDA and contract.
And of course, ask for references if possible but most of all,
examples of previous projects. Do a research such as reverse image
search or looking in the source code to see if the work is really
his/hers. Sometimes a nice Wordpress theme might look very good
but it's still not really about coding but small code customization. That
might not be what you need or can't do yourself.
Make sure the terms, payments, mode of payments, number of weekly hours and conditions are agreed on, and
understood. As contractors, they should understand that you cannot
give them 100% of the payment you get yourself simply because of
your work managing the projects and also the cost of transactions. A
good number, is to keep at least 30% for yourself and adjust this
accordingly. If you ever get into partnership with someone else, then that number can change but so is the share of the expenses.
You should tell your contractor or future partner about the kind of
contract and clients you target. Some people might not be
comfortable working on some topics for different reasons (eg. moral
principles, religious beliefs, previous agreement with their own
You can find freelancers pretty much anywhere and don't limit
yourself to freelance websites. In fact you might find a better fit
in business online groups (eg. LinkedIn), associations, by
referral, local employment center or by browsing the web. On
freelancers's sites, a lot of freelancers have their own business
and enjoy the "freedom" they get there, OR have very rigid terms
that may not be a good fit for collaboration OR are part-time
freelancers and not fully invested in their freelance career. A lot of very talented and hard working freelancers are not always "hawking" loud about their skills because... they're busy working or they simply more introvert! You can find these people sometimes by being observant on social media or following their blogs; they could be a friend of a friend for example. Have initiative and take the attitude of a talent seeker.
Finally, it's the same as hiring an employee and you have all rights and the tools to make sure you hire someone you'll enjoy working with. You can totally act in the same way as any employer, you can set your requirements as you want. You can start by hiring them as contractors and then offer partnership that should give you more freedom if you change your mind along the way or discover things you don't like about the contractor (eg. bad attitude, late projects, bad quality.)