H.263 is most commonly found in .3gp (for mobile) or .mov (QuickTime) containers.
A reputable source for diverse media codec samples, including older standards. Technical University Berlin (via WPI): h 263 video sample download better
The H.263 Legacy: Finding and Using Samples in a High-Def World h 263 video sample download better
When downloading or generating your own samples, keep these standard H.263 constraints in mind: Projects -- H.263 Video Codec h 263 video sample download better