Where can I find Informix IDS version 10?

Meh.

Informix IDS version 10 is too hard to find on the internet - at least in a legit way. You'll be better off settling for IDS version 11.5. You could opt to go for IBM's time-limited trial or go for the one offered by the IIUG thanks to the info here. (IBM also offers a time unlimited version of IDS Developer Edition version 11.5 so you could go that route as well but I was specifically looking for version 10 and ended up at the post I linked to which is from way back 2006). Unfortunately, you might not be able to easily locate the downloads as per the instructions on that post unless you look harder. Here are some tips:

After creating a free IIUG account, re-validate your email address to allow you download from the Software Downloads sections. How you get there? Simple.

Step 1
First you need to be logged in here: http://www.iiug.org/software/index.html. Even after a successful login, the visual feedback still says otherwise, since the login controls still appears.

Step 2
Click on the Member Area link near the top of the page, underneath the login control which should display a popup with the address: http://www.iiug.org/cgi-bin/member_area.cgi in the location bar.

Step 3
Now click on the Software Download button then follow the steps to re-validate your email address to allow you access to Downloads if you haven't done so already.

Step 4
Once done, clicking on the button once more will present you a list of tools/programs available for download. Please be informed that the downloads are subject to terms which you have to agree to.

As at the time of writing this post the following four items are listed as available for download:

Informix Dynamic Server (IDS) Developer Edition 11.50 xC5 released August 10 for AIX (64 bit), HP (64 bit), Sun Solaris (64 bit), Windows(32 and 64 bit), Linux(32 and 64 bit) and Macintosh (64 bit);
Informix Dynamic Server Open Admin Tool Ver 2.24 released April 2009 (OAT);
AGS Server Studio;
AGS Sentinel;

Enjoy!