EPSON does not list the Stylus SX515W as a model with Windows 8 drivers provided here on
their Windows 8 support page, however on this same page they do provide instructions for installing the Windows 7 drivers in Windows 8.
The Windows 7 drivers are provided by EPSON here, the TWAIN driver would be provided under the EPSON Scan package under the drivers category.
The Windows Compatibility Center lists this device as compatible. If you have issues with the TWAIN/ Scan driver specifically, it would be beneficial to report that using the Give us your vote feature provided on the page.