The official Seiko site is:
Software Driver Downloads
However, Seiko has deleted its legacy printer support so you will have to find your drivers on a third party site. I've included links to your drivers below:
Seiko SLP 420 Free Driver Download
SIISLP200 Drivers Download for Windows 7 8 XP Vista
Note: With Seiko Smart Label printers make sure your printer is not connected to your computer until the Seiko software tells you to.
Important tips when finding and downloading application/ driver software from non-OEM sites
1. Never install any application from a non-OEM site that says it will scan your computer to determine what files you need. - search using the manufacturer name, model number, and current operating system on the computer you want to install the device on.
2. Only install the files you found. Never download any software suggested.
3. When installing the downloaded files, always select Custom Installation to make sure you only install the software you need. In this case only the application and driver files, nothing else ( no driver updating, security scanning, toolbar, etc. programs).