I used Buildium software, which has a listing feature, and that lists the units on Zillow, Apartment List, and Rental Path, and then from there other services pick up the ads and repost them.

I also like the north Fort Worth area, with a lot of new construction I feel like the area will continue to increase in value. 

You are correct, if your intention is not to continue with your side self-employment business activity then Solo 401k would not be the best choice for you as you would have to terminate the plan if the plan sponsor ceases to exist.While self-directed IRA LLC does not have all of the advantages of the truly self-directed Solo 401k it would be your option if you wish to have checkbook control over your retirement moneys. 

What other options do I have other than List Source, or should I just continue to build my own database?  

We will continue to allow it to build then buy more properties.

Some people will buy a property subject to the existing financing which will transfer title into there name and leave the loan in the previous title holders name, but the new owner will continue to make payments for the old owner who's name is still on the financing/loan.

It seems that less and less people are going in to or even considering trades, and the plight and challenge of finding consistently good contractors will continue to increase.  

@David Briley I would say that all of those lists work as well; however, you need to make sure that one list you stick to and mail continuously.

We are in a shortage of entry level SF's right now so prices continue to rise and homes fly off the shelves, especially in the 175-290k range.  

As you continue forward I'm glad to be of any help I can with my limited experience.