How do conferences / journals select reviewers?
In the journals I help edit (all in the medical fields), there are five ways we find reviewers for our manuscripts:


They have published with us before
They were nominated by the authors, editors, other reviewers or peers, or members of our editorial board
Our associate editors approached them through a scan of authors of papers on the same topic
They have volunteered to be placed on our list of reviewers
They work for us as consultants and are paid on a per-review basis (this is mostly relevant when we need specialist expertise such as in statistics or economics)


Once you are found, you are placed on a pool from which we can make selections on the basis of affiliation, expertise, seniority, etc.

As you can imagine, this is a rather valuable resource. We guard this list jealously and do not share this with other journals. 

In my experience the response rate is typically 33%. That is, if we require five reviews, we will invite fifteen people. Thus, the pool needs to be deep in order to receive the requisite number of reviews for our purposes.