What types and forms of artwork were produced during the Ancient Rome--Roman Republic to Roman Empire--time period that provide information about the clothing?
When studying ancient art the type of documentation that describe life as the ancients knew it, aside written documentation, is the graphic representations seen in colored painting and pottery. In Ancient Rome the Etruscans were the pioneers in arts and architecture, creating the unique style that is representative of the era.
This being said, most of the documentation that we get from Ancient Rome comes in the decorations that are mainly found in vases. Imitating the style of the Greeks, the Romans also stamped in their daily use pieces imagery of daily life, complete with pictures representing their diet, their clothing, and their way of life (hunting, wine-making, dancing, worshipping).
Along with pottery, lest we forget the detailed sculpting that came out of that era, which also depicted the clothing of soldiers, citizens, the elderly, and senators. Anyone who could afford a commission or was worthy of remembrance would have had a sculpture dedicated to them.
Painting was yet another point of interest for the Romans. They mastered depth and perspective in their art and added dimension to typical works. As a primary source, paintings are a rarely found but highly documented practice among the Romans.
The picture below shows an example of late republic Rome circa 40-30 B.C where the clothing is clearly described to the most minute detail.