I use units from APC. I have a 350 VA (200 watts) unit which is enough for my modem and routers. There are other UPS makers. Look for what is available locally and buy on sale. With the heavy batteries free shipping for a UPS is difficult to find.
The size UPS you select will depend on what you want to accomplish. What you stated you are looking to do is increase the stability of your network equipment by reducing fluctuations in the input power. In addition you will get a few minutes of run time for your modem and router before they blink off when the battery draws down if you select a 350 VA unit. Buy a 550, 750 or 1000 VA unit if you want longer run time.
If you have a laptop with a decent battery and a larger UPS you might be able to surf the web for an hour or more.