I need to integrate an API created with .NET with SAP FI/CO.
Researching online I found out there used to be something called IDES that would provide access to an SAP server free of charge, just for testing purposes, but that's no longer available.
The Consolut solution seems to have been a viable solution while it was active: https://www.outsystems.com/forums/discussion/6598/sap-server-test-account/
Anyway, my question is: Is there any way for an independent developer to test an SAP integration without spending money to access that server? Are there any options out there that would allow me to access an SAP instance for testing purposes?
Here you can find the official list of developer trials that you might access without having any kind of relationship with SAP: https://www.sap.com/developer/trials-downloads.html.
If you have an SAP S-user, you might have access to more stuff from the SAP Marketplace. From my knowledge, IDES was / is only accessible through the SAP Marketplace (or external sources which use the marketplace themselves).
FI/CO runs on Netweaver AS ABAP (or an older alternative, e.g. R/3), so you could download it here: https://tools.eu1.hana.ondemand.com/#abap. But it will not have FI/CO in it. Based on the discussion here https://archive.sap.com/discussions/thread/2039981 I would say that it is not really possible to get FI/CO up and running on your local Netweaver.
This is a general strategy of SAP. They provide the possibility of trying out their technological platforms (HCP, HANA, NW, etc) but they don't really offer access to their business solutions for free.
If you don't find any other way of getting access to a FI/CO system (e.g. from a third party or customer) then maybe the best thing would be to use the local Netweaver and mock the FI/CO specific RFCs that you are using (assuming that you use NCo for the integration). At least this way, you would know that the integration works at a pure technical level (at some point though, you will need a real system).