There is an Android app RealCalc that can work both in RPN and standard calculator mode.
I use an HP48 app. I suspect I might have lost the ability to use a calculator without RPN. (Although I don't know for sure because I haven't tried.)And there is Free42, which I use almost every day on my phone at work...
I have an HP-67 that was handed down to me by my aunt. Unfortunately it no longer works. I am hoping I can get it repaired some time.
I bought one of these that I saw in a shop about a decade ago. HP introduced it in 2007 to commemorate the 35th anniversary of their first pocket scientific calculator, the HP-35. It has been in continual production since then, presumably because it's one of only a few scientific calculators allowed in some professional engineering exams in the US.They made a retro tribute to one of the HP calculators a few years ago. The guts are modern but the UI is the same classic design.
I don't think you're limited for choice if you want to use RPN on a computer. The dc calculator (one of the traditional Unix command-line utilities) and Forth programming language are both stack-based and use RPN notation. I am not sure this is so useful in a programming language though. RPN to me seems great for inputting and doing calculations interactively but not so good as far as readability is concerned, which is usually something you want in a programming language.There is an Android app RealCalc that can work both in RPN and standard calculator mode.