But how does one go about finding these quality agents, that know the areas, in towns and outskirts that aren't popular or populated?

If I were starting out in this market I would look in other markets in AZ that has a large population and job growth like Casa Grande or other cities that have lower price points than phoenix and less investors that have more money and more marketing dollars than you since they'll get the good ones before you if they touch the market. 

I would like to use a similar letter but not sure how to replicate this with a mass mailing and have the correct offer to auto-populate for each address.

There's such a killer rental market here that we don't target a specific population...there are plenty of folks who want to escape their home renovation for several months, place the in-laws close by but not too close, and more. 

But the single most important factor of any real estate investment is location.Investing in a small town might be ok but you don't want to invest in a town with declining population, no jobs, etc.My point here is just pay attention to this factor.

Giving ourselves the opportunity to build a more accurate expectation of NOI Initial expectations for COC is about 7%, which isn't amazing for a STR, I know, but we're also looking at a 2X equity multiple and a 10% IRR, but with a lot of room for upside on market appreciation (market has been the fastest growing in its MSA over the past 10 years and shows no signs of stopping in terms of population, and job opportunities, and long-term price growth).

Things such as: population growth, median income, supply of other storage facilities, and demand at those storage facilities. 

Population and demographics is a big player in your formula.

Tradoc (Training bases) These have a large short-term (3-9 month) population and a small permanent  party (3-year) population. 2 Forscom (regular bases) all most 100% permanent party population. 

The population growth here has been incredible.