After using the 300 MB of free storage space, Zotero offers options to purchase additional storage as a subscription. There is an FAQ page offered by Zotero on the topic of storage.
If you are looking integrate Zotero with other external storage options, check out this “alternate storage?” topic in the Zotero Forum.
Zotero is not the only option for citation management. Here is a table (courtesy of Wikipedia) comparing features and price points of several other notable citation management systems.
