Would your users have to type the same street name multiple times? Because you could easily prevent this by coding something that prefilled the fields.
Another option could be to use your user database as a resource. Query it for all the available street names entered by your existing users and use that to generate suggestions.
Of course this would only work if you have a considerable number of users.
[EDIT] You could have a look at OpenStreetMap with their Planet.osm dumbs (or have a look here for a dump containing data for just Europe). That is basically the OSM database with all the map information they have, including street names. It's all in an XML format and streets seem to be stored as Ways. There are tools (i.e. Osmosis) to extract the data and put it into a database, or you could write something to plough through the data and filter out the street names for your database.