Most people prefer to use participants they know, largely because of the comfort factor

• If you’ll be attending a fetish convention (or similar event), you might be able to recruit participants there.
• If you’re aware of local sex boards, you can use those.

If you know everyone, there’s less awkwardness involved and you can be a little more confident that everyone will follow the rules.

Using randoms will provide you with a much larger pool of potential participants and therefore make it easier to recruit sufficient numbers.