I couldn't even find much about them on ebay. That suggests either it's so popular and durable that no one wants to part with theirs, or it wasn't a very popular model, and they sold very few of them.
You could contact Sony support for the location of authorized repair facilities, but I'm afraid going that route could cost more than buying a new machine.
I'd think the reasonable alternatives would be to search Newegg for similarly equipped new machines with Win 7. That would keep your dad on the same OS, but he'd have to learn a slightly different layout, but not THAT different because most laptops are pretty similar, and it would have a new machine warranty.
For my links, I narrowed the search to machines with an i7 You'll find an even greater selection if you can broaden your choice of CPU's to include an i5 and/or upscale AMD CPU.