The proper search term would be “Standard 10Pin to 6Pin Adapter Board For ATMEL AVRISP USBASP STK500”. That got me 30 hits. I prefer the smaller LC Electronics design (widely copied) that is a blue board using the smallest 6 pin connector possible. There are some black and green versions that use a larger than necessary 6 pin body which prevents its use on a board where there are other components or pins near the ISP connector.
I also recommend the usbasp units with the plastic case, much better long term than a bare board. Here’s a combo set from a well known and reputable (IMO) seller:
It could be said that the enclosed version has the disadvantage of being difficult to switch to a 3.3v target voltage. I would agree but for pro mini’s and similar boards, it doesn’t matter as AVR’s can flashed using a 5 volt supply and then operated at 3.3. The only caveat would be a custom board that had 3.3v only devices.
Ps: do not purchase the units sold as USBISP/USBASP in the aluminum cases, they are not for Arduino use but rather the older Atmel 8051 based products like the 89C52 and similar pre-AVR parts. I had zero luck with within the IDE as the USBISP but they do work with the older Atmel 8051 IDE. Typical Chinese description “creep”.