@Stanley Reader Assuming you are wanting to buy post-sale tax liens, each state has its own unique way of issuing tax liens or tax titles.
Additionally, a sellers premium is typically added to post-sale tax liens which increases the price of an already risky investment (risk being no assurances you will hold deed to the property).
As a result, the post-sale tax lien marketplace has not been organized into a single online portal. I see this as a strength for the focused investor because it prevents institutional buyers from hording all the tax liens with block purchases.
ANSWER: Rather than looking nationally, you should look local. Many counties only sell their adjudicated or abandoned properties over the counter. Visit each county office responsible for selling or managing unpaid property taxes to get the process started to take possession of them.
For example, in Louisiana, I had to drive 60 miles to the Cameron Parish Assessor's office to get a list of adjudicated properties. No such list exists anywhere online and it was filled with great outdoor properties that were then sold to local hunters at a major premium. The real bargains are not always homes or online tax liens but rather those properties hidden in the tax rolls of the county next door!