Trademark Clearance Searching
There are two things you are searching for – i) federal trademark registrations and applications in the US, and ii) common law trademark rights acquired through use (unregistered trademarks) in the US.
For i) federal trademark registrations, the definitive list is in the USPTO TESS database. Search by the trademark itself, and by variations such as misspellings (or the closest dictionary word) as well as words that sound the same and mean the same. The goods and services must be similar for marks to be considered confusing, so you will also want to consider these for every mark that you see as close.
For ii), unregistered trademarks (sometimes called non-registered trademarks), you have to see if the product is being sold in the US, hence search engines like Google, Bing, DuckDuckGo, and online sellers like Amazon, Walmart and eBay may be helpful. Look for use of the mark and the product’s availability within the US. Even if the mark is not registered, if the product has been sold in association with that mark in the US it will acquire common law rights which can be asserted against your mark down the line. The common law rights generally expire after a period of non-use, so only relatively recent use should be considered important.
Spending a couple of hours performing a clearance search, before you file a trademark application, can save you many hours of time rebranding or starting the trademark process from scratch if the trademark name is already taken.