Actually, the word is "styli" (plural of "stylus"). Admittedly there are
some vendors that spell it with two I's for unknown reasons -- maybe to be
more Latin than the Latins.
There are several good online vendors of styli. I suggest
www.needledoctor.com and
www.lpgear.com.
Another that looks good is
http://www.turntableneedles.com/. Type "Dual
506" in their search box and you'll get a page of pictures of styli and
other components.
What make and model of *cartridge* do you have in your Dual turntable? I
gather, from a quick web search, that the Dual 506 was sold with many
different kinds at different times.
Also, if it's not 1/2-inch mount, is it by any chance P-mount or something
else? I'm not familiar with that Dual turntable.
One thing you might do is take a picture of the cartridge and/or existing
stylus and either post it on the web (for us to see) or send it to the
stylus vendor. You can even photograph a stylus by laying it down on a
scanner.