Hello Ken,
Congratulations on owning rentals in the Castleton area. That is a strong area to be in and you should have no problems finding good tenants if screened properly. You are correct, the Indianapolis market has tightened significantly over the last several years. I started buying here at the bottom of the market when everything was in foreclosure and pre-foreclosure. Great deals are harder to find in this market, especially in stronger areas. I would say that you lucked out with your neighbor if you bought that property right.
You can look at local wholesalers, but will probably not find their inventory in better areas like where you are invested now, and conventional financing will probably not be an option, depending on condition. In that case, you will be managing properties in areas with higher turnover and more tenant issues.
You can get with a local realtor who knows the investment climate in the area and set up an email notification for any new MLS listings hitting the market that meet your specific criteria. You will have to be prepared to act fast after analyzing the property and determining that it is a good deal, or pay current retail for the property in areas of likely appreciation. You can also look at properties that are about to expire off the MLS to find motivated sellers. PM me if you want to talk further about this.
You can "drive for dollars" and send out mailers to property owners, but that is time consuming, and may involve a cash offer to get the deal done. Networking with other investors at local investors associations such as CIREIA can also be helpful. There are a lot of people out there trying to do similar things!
Good Luck.