I don't know any modelling archives for Deus Ex except this one:
http://www.planetdeusex.com/endemia/
If you really want to learn modelling (or at least how to work with them) you have to learn MilkShape3D
http://www.swissquake.ch/chumbalum-soft/
More about adding custom meshes here:
http://www.planetdeusex.com/tack/HowToA ... eshes.html
Sources of obtaining models for you mod:
0. Ask other mod (like Cassandra, Redsun, Zodiac and etc) authors for permissions and grab models from there.
1. Grab models from other games (you can find a lot of plugins for conversion on MS3D page) - most of mods do that a lot.
2. Grab models from model archives. There is a lot of in the internet. Google is your friend in this case.
3. Make your own model. Yes, it is not easy.
Hope that helps.