Generally speaking, dress sporrans (as opposed to those worn with day wear Highland attire) are fur or hair. Both goat hair and horse hair sporrans are enjoying a revival, and seal skin (if you can find it in the USA) is always a good choice. What makes a dress sporran a dress sporran is the cantle-- silver or silver plated is the standard, with pewter also popular. Full mask sporrans are also a good choice, but one has to be careful that the full mask sporran isn't so large that it overpowers the rest of the outfit; with the exception of goat and horse hair sporrans, anything over six or seven inches in length is probably too long for formal wear.