Not many pieces of computer hardware are still in high demand after 30 years, but the Model M is a notable exception when it comes to this. The clicky tactile switch that they are known for is still highly sought after, and in general even used Model M's in somewhat rough condition can fetch a good price. You do have options though:
You can purchase a new Model M keyboard - IBM sold its keyboard manufacturing factory in the US to Lexmark in the mid-nineties, and later Lexmark sold the patents and tooling to some of the employees who formed a new company, Unicomp. Today you can purchase a new "Model M" from Unicomp, as well as replacement parts for your old ones if necessary. The model that most approximates the example shown in this tutorial is the Classic 101 Buckling Spring PS/2. I'm not sure what the controller looks like inside of these or how easily it will be to wire up the PS/2 terminals to the Arduino, so caveat emptor.
You can go dumpster diving - The Model M was a very prolific piece of equipment, so if you are adept at pulling electronics back from the abyss you might be able to find one. Businesses upgrading there systems or schools/universities that are cleaning out their closets may be good options. Also, try checking Craigslist for free or inexpensive listings of old IBM PC systems, or check your local thrift stores.
You can purchase one in used condition - I would definitely recommend going this route for this project, as you will be able to save some money versus a new or refurbished keyboard, and also you can make sure that you get one that is easy to work with. If you are buying a keyboard off of eBay, you might want to verify the condition and make sure that the pictures they provide are of the actual keyboard you are getting. I'll be working with the rather rough looking specimen show below, which is definitely in need of some TLC.