I have other deals where a pay principal only payments over 84 months. 

40 years of real estate probably closing in on 300 million or more in transactions that I have been a principal in  thousands of loans that I have made with my own capital. owning 400 + rentals...

, how do many obtain the principal to buy their 1st investment property after buying the property they reside in?

Could be used as a principal residence or a vacation rental.

When you do your property analyses there are 6 major items to include:  principal, interest, taxes, insurance, maintenance, and vacancy.  

Save that money so you can buy another investment property and get more of that Cash on Cash return in the 30 to 40 to 50% (if you add rental profits, appreciation, and principal paydown). 

For example, if it's $200/month, you are looking at $2400 negative cash flow a year, so it will take 16.5 years to loose $40,000, while you will be paying a lot of principal down and you'll get some appreciation.  

Looks like you have 27.5 years left on your mortgage, so take a look at your amortization schedule to see how much interest and principal you are paying each month.  

The bird dog then gets the property under contract and assigns it to me with a $1,000 assignment fee tacked on, making him a principal in the transaction.

Some commercial RE guys that are principals at mid size firms.