You can purchase from PTC a subscription to their eLearning products. This includes access to PTCU which contains quite a bit of Arbortext training material. It is delivered as self-paced online learning modules.
Alternatively, Arbortext ships with some useful documentation such as the Arbortext Customization Guide. You can search for documentation either in the Arbortext Help Center, that ships with the product, or on support.ptc.com.