In Holland this kind of insurance is required by law to be included in the rental agreement.

I found two possible solutions: 1) get a yearly rental-car insurance (around 100 euros a year, covering any number of rental agreements) that provides super-coverage for the 600-1500 euro co-pay and some extras (window damage), e.g. at https://www.huurautoverzekering.com/. 2) get a credit card that offers extra rental car insurance that covers the deductible

I opted for #2 since I'm welcoming the upgrade in my cc for other reasons too, and the yearly cost was less for my specific needs. My new cc doesn't fully eliminate the deductible, but it decreases it by 500 eur - good enough for me.