I always remember that rockstars know rockstars, so find someone who is a mover and shaker - likely a meetup host - and then ask them who they know.The people doing 100 flips are super busy, so I've also found the best person to find is someone who is a step or two ahead of me...so maybe for you, that is someone doing 2-3 flips a year. 

I also have $100,000 in the bank, and I'm trying to figure out what steps to take next.

I'm inferring from this post that your marketing strategy has a lot of... steps... to it. 

@Bob Asad I like the individual sub account method escrow account, but there are multiple different ways to hold security deposits and multiple laws in different states.

Besides registering with the city, are there any other steps required to setup as a STR/Airbnb host in Alexandria?

- Are there specific tools, resources, or methods you use to simplify the regulation process?

Aside from what everyone else wrote, note that adding units require various steps that you may not have considered (e.g., adding a new street address with USPS, etc.).

Joseph, the best method for you would probably be the MLS or sites like realtor.com etc.

What should my next step be?

These larger projects are going to have a lot of steps.