The most important part of repurposing a calculator to become a WP 34S is to have the 'donor' calculator. In this case, you'll need either an HP-20b or HP-30b calculator. Either calculator is fine for repurposing; it's primarily a matter of look and feel on your part as to which calculator you like more. The HP-20b is shiny black and less expensive, while the HP-30b has a brushed silver look, arguably a much better keyboard and slightly better display. It's also a bit more expensive (usually US $10, on average).
Otherwise, both calculators are essentially the same in terms of their hardware. Here are some links to places to purchase these calculators:
The WP 34S firmware is open-source software created primarily by Walter Bonin and Paul Dale. It is freely obtainable below.
Download the wp34s.zip file at the green button location on this page:
Unzip that file into an appropriate directory. The actual calculator ROM image is in the “real” subdirectory, and will be titled “calc.bin” in all cases.
If you attend an HHHC conference, chances are good that someone will “gift” you one of these cables for free. If you're unable to attend, try politely asking Gene Wright and he may be able to get you one. Similar cables to these are available for purchase online (links??) if everything else fails.
In order to program (or “flash”) the calculator, you will need a copy of the programming software. The software needed is available here, for free, from Amtel, the manufacturer of the CPU in the calculator. Download this software and install it.
Complete and full instructions are available here. Note that these were written for the HP-20b, but are fully applicable to the HP-30b.
The basic steps in flashing your calculator are:
A user's manual is under development, but you can get started here for now.
Eric Rechlin has created an amazing keyboard overlay for the WP 34S. It completely changes your calculator. You can see details and pictures here, and contact Eric for ordering information. The overlay looks a bit better on the HP-20b calculator due to the black casing and the black overlay background, but will work with either calculator model.