1. Nope. Various sexually transmitted infections—gonorrhea, syphilis, chlamydia, herpes, HPV, etc.—could be contracted by the extra person and/or passed on to you and your fiancé. There's low to no risk for HIV, PUSSY, but the act nevertheless falls outside the realm of safe sex. Very little actually exists in the realm of purely safe sex. There's always risk, we can mitigate for those risks, we can make sex safer, but save for solo and cyber, sex is rarely ever 100 percent safe.
2. This is technically three questions, PUSSY. You find that person by putting ads on hookup sites and/or by putting yourselves in places where you might meet that person, i.e., pick-up joints, sex parties, swingers clubs. There are lots of apps out there for couples seeking thirds, you can even advertise as a couple seeking a third on big dating sites like OkCupid. It is a degrading thing to ask someone to do—but since there are lots of people out there into erotic degradation, that's a potential selling point.