I like breeding animals because who doesn't want a 1000 strength songbird? Anyway, I put together a program to find folks selling animals which are still able to breed and have a usefully high level via vendors.uoex.net:
http://www.udic.org/fallible/breeders.cgi
Instructions:
1. Go to vendors.uoex.net.
2. Search for "A Shrunken Pet" or a specific pet breed such as "Breed: Mule".
3. Copy the entire web page (Edit->Select All, Edit->Copy) and paste it in to the form at http://www.udic.org/fallible/breeders.cgi
4. Click "Sort"
I also sell a lot of breedable pets on the vendors at World Teleporter -> Public Moongates -> Britain. If you see a pet on display but not on the vendor, I have breedables of those as well and am usually willing to sell them. Just PM me.
Also for sale: Poor Man's Pet Trainers. 10k-50k gold depending on how good they are. Basically, these are birds with high hitpoints and resists but no ability to do damage. You can level your weak pets against them at negligible risk. Much cheaper than a real personal trainer (500 ED).
-Wil