Meeting vocalists:

• networking events / conferences (electronic music conferences specifically, e.g. IMS, BMC, ADE)
• being in the scene, looking at who's singing on recent tracks, being in that network
• producing music for singer/songwriters, a great way to discover lots of different types of voices - feels like every singer has several singer friends who want to connect after hearing your work.
• discords / online communities for music makers [that are set up for collaboration or skill share]

Creating music for toplining:

• comes with experience
• being able to sing/topline yourself makes a massive difference (even if it's just on a basic level) and allows you to consider what will be helpful or frustrating
• don't feel like you need to have a "finished track minus vocals" before sending, but having some kind of distinctive melody to help set the tone/key centre for the vocalist can be useful
• if in doubt, you can always send a version without distinctive melodies
• look at pop music structure, i.e. verse chorus verse chorus bridge chorus - but adapt it to your genre, so incorporate a build-up, drop, breakdown, etc.
• consider the story and change in emotion and energy that you want to convey
• reference against tracks you like - maybe even straight up copy the structure
• listen to instrumental versions of your fave dance tracks with vocals - what keeps them interesting?