I've been around real-estate for years, because my parents and maternal grandparents own multiple properties. I spent some time running away from real-estate, because I couldn't stand dealing with the constant maintenance headaches, nosy neighbors, nazi city-inspectors, and crazy tenants. (In fact, that experience helped to push me to be a renter-for-lifer for a few years.)
As for the tax liens, let's just say that's how I got my feet wet getting back into real-estate on my own (this time by my choice).
Each county government has its own way for handling tax liens and sales, so there isn't a single source online or otherwise to go to for obtaining them. Some counties (like Maricopa County in AZ) have highly organized records, and a streamlined process that you may access online or via phone. Other counties, . . . you'll need to make a few phone calls, some more calls, and some more calls, before you reach the right person for information on their tax liens and sales.
There is a caveat: some states are tax lien states (like AZ), and others are tax deed states (like OH). Both issue liens that one may purchase, but the process for collecting on the unpaid taxes differs between the 2. In general, tax deed states typically require the lien holder to participate in a tax sale either to reclaim the deed via an auction, or to reclaim the back taxes from the proceeds of that auction. In general, tax lien states can be further classified into 2 groups: judicial or non-judicial. The judicial states require lien holders to go to court and file a lawsuit to reclaim the deed or back taxes (which the primary mortgage lien holder typically will pay in order to take over your position). The non-judicial states require lien holders to simply report the fact that the taxes remain unpaid, and don't require any court action to reclaim the deed or back taxes. Also keep in mind that the details varies (sometimes wildly) from county to county.