All rental car companies in the US will offer Liability coverage, generally referred to as Supplemental Liability Insurance (SLI) or something similar. This will be priced as a per-day rate in addition to your rental car price, and you can elect to take the coverage either when you pickup the car, or sometimes in advance on their website.
However given you are from Europe, you will likely find that if you rent through a Europe-based company - including the European websites of the US companies such as Hertz and Avis - that SLI will be INCLUDED in the rate (along with CDW/LDW), almost always at a far cheaper price than paying the US rate.
For example, I just went to the Hertz UK website and attempted to reserve a compact car for a week from San Francisco airport, and got the quote below - note that both Loss Damage Waiver (LDW) and Liability Insurance Supplement (which for Hertz generally provides $1 million of coverage) are included in the base rate! And all that for $6 cheaper than booking the same car on the USA website - without any insurance!