You need to be more specific about which Sage system you're trying to integrate with. Assuming you're talking about The Sage Group plc business management software, there are still many systems included under that umbrella. (Sage MAS90, Sage ACCPAC, Sage ERP X3, and many more.
It sounds like you just need to read data from whatever system it is that you're using. Given that assumption, if your Sage system stores its data in a RDBMS, you may be in luck. In that case, you may be able to do a little reverse engineering to figure out where the data you want to display resides. Then simply create a service to query the db for that information.
Some of the Sage systems use flat file formats, which may even be proprietary and will probably be hard to figure out how to find the data you want. If this is your case, hopefully they will be text files and you will just have to try to look at the files and see if you can make any sense of them.
Another option is to consult with the reseller you bought the system from. They may have the expertise to help. Or you can contact Sage directly and they can refer you to a reseller or development partner or whatever they call people from their approved list.
And for the obligatory disclaimer, I am not associated in any way with Sage Systems.