I wrote my first program on an HP-25. I was fascinated with RPN. My first calculator was the HP-67, which I got as a present in the summer of 1977, but had to sell it in the winter of 1980 so I could afford an HP41C (I was a poor high-school student them). That HP41C acquired a card reader (82104) and a memory module (82106), which got replaced by a quad module (82170) shortly thereafter. The poor thing lasted for three years, when it died an untimely death (static electricity is your enemy), and got replaced by an HP41CV, which subsequently acquired an X-Functions, X-Memory, and Time Modules, as well as a Circuit Analysis pack. A wand joined them in 1984. They got stolen from my desk when I was working in France in the spring of 1989, and I have been lamenting them ever since.
I have been recently acquiring HP41 stuff on ebay. An HP41CX with an X-Memory module, an HPIL interface and an HP82162 were the first purchase, followed by an advantage module. I also have a resurrected HP41CV which I use for experiments with an MLDL I am designing, so as not to risk the CX. Another HP-41CV (with good battery contacts), a pair of 82104 card readers (one with a gummy wheel, repaired, given to a friend), a wand, an 82164 HP-IL to RS232 interface, a 9114 floppy drive (!), and assorted modules, give me enough to play with.
I also own an HP-35, an HP-45, an HP-67, an HP-67, and an HP-67, all of which I got on ebay.
The Museum of HP Calculators is an excellent resource. Look in http://www.hpmuseum.org/links.htm for links to other sites.
Thermal paper is still available!