models...how do I find them? and related questions

Look for photo clubs in your area. They often have contact to models. Use social media. There are also often pages listing amateur models. Advertise. Ofer photos for modeling. Amatuers often accept these terms while they build their own profile.

1)  You will need to ask around. Friends and friends of friends. Offer them something in return, like a free portrait shooting.

2) Well, you tell them the truth. You are looking for character models and that they would nicely fit in. You shoot for stock, and they could end up on a book cover, in a magazine or on a billboard.

3) There are model release apps, but the easiest is to have model release templates on your iPad.

You should also have business cards, if they would rather not decide immediately. They can contact you later, and you could make out a model shoot.

I've never used it, but there's a site called Model Mayhem, where amateur models register to make themselves available to photographers. If you live in a large city, you might find some that are available in your area. Prices vary based on their experience, while some will model in exchange for physical or digital prints to add to their portfolio. I can't attest to the legitimacy of the site, but I haven't come across any complaints about it.

I no longer shoot photographs on an active basis, but I used to approach strangers at malls or other public places while having my portfolio handy. This was many years ago and, being male, this approach would no longer fly in this day and age. But since you are female, it might work for you if you have no problem approaching complete strangers.

And yes, always keep some model releases handy for the possiblity of impromptu photos.

I was also fortunate in that I was a computer graphics and photography instructor at a college, so I could usually find students who were willing to be of assistance (though they were not allowed to be my own students at the time).

In any case, be cautious. I once photographed a model in exchange for prints and 24 hours later, I got a call from an employer  (unrelated to photography) asking me about her background. She used me as reference, though I only worked with her for one hour.

I can't answer your first 2 questions because I haven't hired models for photo shoots, nor approached strangers to get permission to use their images. With regard to question no. 3, some photographers use the "Easy Release" app which you can use from your phone so that you don't have to carry around a paper form.