You can buy Win98se here at
newegg . However, stores will require that you buy some hardware because of rules that Microsoft made. A floppy drive,hard drive, etc.
You might also take a look at your local 'buy&sell', newspapers, etc.
The thing is Microsoft is not going to support 98se any longer. For the price difference, you might as well pick-up XP Home.
If you need advice building a system, I've built 5 or 6 and I'm sure others on the board have as well.
Good luck.