Amost any small UPS 375 or 500 VA, will support a broadband modem for a couple of hours. Check your local office supply store - you're in the $100 price range.
You can even go by weight - if it's heavier, the extra weight is in the batteries! (assuming similar cases - metal or plastic!) Many home or small office UPS's are in plastic cases now. Check the specs to see how many Ampere/Hours the battery is rated for. The UPS specs may also say how many minutes it's good for at a specific wattage load or current drain. You can extrapolate that linearly - figure minutes times watts or amps as a constant, so you reduce the load in watts or amps, you get more minutes. Figure out the wattage load on your broadband modem (see what it says on the power brick for it), and you can predict quite well what run time you'll get with a particular UPS capacity.
Fred Wagner