You could try one of the free Oracle or SQL Server Express editions - but they do have limitations (e.g. max DB size ~10Gb etc).
If you are unfamiliar with them, I'd suggest SQL express on a windows server (or your workstation) as it will be simpler to get going. If you are using the free version of Solarwinds, it won't need much disk space as it won't be keeping all the history.