Re: Constantinopolis commemorative coins (cheap)
I've never heard of anyone going into an in-depth collection of these but it sounds like a great idea. There are a ton of differences. Over a dozen mints and a few less common varieties like the GLORIA EXERCITVS reverses and YRBS ROMA medallions (not that these last are affordable).
The best place to load up I would think is ebay. They only cost a few dollars each.