Here you can find some online resources that may be of use in your research.
Finding Numismatic Literature
All numismatic articles can be keyword searched on the American Numismatic Society website. Use this website to obtain information about what particular articles exist on a topic.
Another good place to look is the L'Année philologique website.
Every six years a Survey of Numismatic Research is published, listing all numismatic publications by subject area. This can be found in the library [CJ 55.S88], but the most recent survey can be found online here. (TIP: keyword search the pdf rather than reading the whole thing!)
Finding Coins and Coin Images
Several websites and museums have coin images available online, allowing you to examine particular specimens and obtain images for your research.
Standard reference catalogues
Coinage of the Roman Republic Online, CRRO (based on RRC reviewed)
Roman Imperial Coinage (volume V, AD 268-276)
Roman Provincial Coinage Online, OCRE (volumes IV, AD 138-192 and IX, AD 249-254)
The Severan Database Project (Provincial Severan types, and frequency of Severan silver imperial coinage in hoards)
Online Coins of the Roman Empire, OCRE (Augustus to Pupienus)
The Portable Antiquities Scheme (HINT: look for the numismatic specific database on the left hand side)
Coin Finds and Circulation
Nomisma maps the coin hoards listed in the Inventory of Greek Coin Hoards.
The Portable Antiquities Scheme provides data on finds in Britain.
Numismatic Exhibitions Online
Several museums and numismatic institutes have online exhibitions. Learn more about ancient coins on the following websites:
Anglo-Saxon Art in the Round (Fitzwilliam Museum)
Between East and West: Influence and Change in Coinage (Fitzwilliam Museum)
The Coinage of Julius Caesar (Australian Centre for Ancient Numismatic Studies)
The Coinage of Ephesus (Australian Centre for Ancient Numismatic Studies)
on Roman coins (by G. Depeyrot, CNRS France)
on Late Roman and early Medieval coins (by G. Depeyrot, CNRS France)
White Gold: Revealing the World's Earliest Coins (Israel Museum) (Tip: Watch the video, its great fun!)