The French National Library's collection is an excellent free resource, with plenty of great plates and rare types.
https://gallica.bnf.fr/ark:/12148/btv1b53200203v/f1.item
I highly recommend you buy the book and support the author, but I also believe that this information should be EASY and FREE to access.
r/AncientCoins & r/ByzantineNumismatics are invaluable resources in every way relating to Byzantine coins, and although bordering amateurish, they are still a great place to converse, ask questions, and discover new information.
Although many numismatists (Myself included) recommend against buying coins on eBay due to the vast presence of fakes, this is my own personal eBay store in which I can guarantee authenticity.