How do I make sure my emails to people don't end up in their spam folder?

Not really. It is dependent on their side.

To some people this is considered spam. Especially if what you are sending does not interest them. You made it clear when you said ‘potential sources’ that you do not know the people to whom you are sending, and they are therefore targets of spam. (At least from their perspective.)
The issue is not about bulk messaging here, but it is rather about them receiving emails in the form of advertisement (to some degree.)
They probably put it in the spam folder for fear of receiving more similar emails.

So as @marinelife said, it is up to them really, they create the folders and the emails are stored there accordingly. What you can do however, is add a little note within the email that makes it clear that you are not a spammer but rather a person searching for ‘potential sources’.

Gmail is a complete piece of crap.

If you depend on e-mail, I would suggest getting an e-mail provider that knows something about commercial e-mail. Proper configuration is key, and gmail probably has that. IIRC, they don’t have some fairly obvious features like DKIM though.

(There are major privacy concerns with gmail anyway. It’s one thing to use it selectively – I use it for people who I know are going to spam me, or for sending stupidly big files – but it’s not a good thing to use generally.)

You can’t. People can adjust their email spam filters. There is nothing YOU can do to un-adjust someone else’s.

Make sur e it is addressed to the individual.
Don’t use all capitals.
Don’t use oversize, colored font
Make sure you have something in the subject line
Make sure you have some text that goes along with the attachment
Don’t have the words: cheap, pharmacy, viagra, manhood, breast, or enlargement, appear anywhere in the subject or text.

That’s about the best you can do.

^^ Nor super weight loss pill supplement.