Thanks for the kind words All.
On a related note: Here are some NOT created by me. They are all freebies from the net (links to homepages below) that are 16x16 sets. I have only converted to .ico format and also ensured that they have 24x24 and 16x16 icon sizes. They are zipped-up and posted on my google drive. Rather than including the read_me.txt of each set, I've appended the name of the artist to the name of each icon.
https://docs.google.com/open?id=0BwtnO5 ... mtjTmdLeGM
http://p.yusukekamiyamane.com/ (Fugue set)
https://docs.google.com/open?id=0BwtnO5 ... 1NneDZYYTA
http://www.famfamfam.com/lab/icons/
https://docs.google.com/open?id=0BwtnO5 ... FIyUkZvVE0
http://led24.de/iconset/
https://docs.google.com/open?id=0BwtnO5 ... DAzZEtYaWc [edit: see below]
http://www.fatcow.com/free-icons
I think that the Fatcow set might be the best... There are 3000 different images. Not as many as the Fugue set, but there is a nice variety of different things. Also, the Fatcow people have provided two versions of each image... There is the low-detail 16x16, and the higher-detail 32x32. I converted the 32x32 to 24x24 ICOs. You'll notice that there are 6000 icons in the folder. I didn't know how to batch-combine the 16 and 24 icos into multi-icos, so I appended "16" or "32" on the end of each. That way if you change the size of your TB, you can just change that part of the path. E.g. "C:\...\star-16fat.png" gets changed to "C:\...\star-32fat.png." As it turns out, having different ICO files for each size might be better anyway. The way I'm reading the help file is that XYplorer looks in the ICO file and picks the first sub-image it finds, then scretches it to fit (hence it would it would use the same image for 16 or 24 sized TB buttons.)
EDIT: Please note that one or more of these sets are listed as "free for personal use only" via Creative Commons lic. So if you want to use them for commercial purposes you should see the artist's website.
EDIT 2: I think the XY help file might be mis-worded.... It states,
If you point to an ICO, ICL, EXE, or DLL file then the first contained icon resource will be used on the large toolbar. Optimal results are achieved if this is a 24x24 icon resource (note that in multi-icon resources always the first icon is taken and scaled if necessary!). Other sizes are scaled. For the small toolbar this ICO file's small system icon is used.
I did some experimenting with home-made .ico files, and if an ico contains only 16x16 and 24x24 images, then XY will always use the correct one for small and large toolbar buttons. And it doesn't matter what order they are in. If the .ico contains a 32x32 though, it usually will get squished, and the 24x24 won't even get used. As long as the icon contains a 16x16, however, the 16 will always get used when the toolbar is set to 'small.'
EDIT 4-22-13: I just realized that when I posted these, I didn't have the technology to combine icons of different sizes. The Fatcow set came as different sized, separate ICO files (as in the above download). I've now combined them into single, multi-sized ICOs, with the help of the awesome bulk tool in Realworld Icon Editor

The combined sizes are 16 and 24 -- those preferred by XY's toolbar.
I've put the combined Fatcow, and the above other three sets into this zip.
https://docs.google.com/file/d/0BwtnO53 ... sp=sharing
The zip also contains some icons that I made, and a bunch that were bulk extracted from Windows. The Windows ones contain different sizes. Some look better than others.