Behance is the first place a design student usually goes to upload their work so could be good if you are looking for new grads. Again there’s just so many profiles on there.
Creative placement agencies are another option as they usually vet their talent first.
Best option is network and find people you can trust through referrals.
2. For me it’s usually about the industry and project. Designers are really passionate about trendy design techniques so anything that allows them to use their typography and illustration skills will be favored over web banners, product manuals, etc. Also anything prestigious or high budget that could become a hallmark of their portfolio would be a big bonus since the designer has to attract more clients once the project is done.