any 600 va or above will meet your needs. why order online when you can buy in stores.
intex, luminous, sukam, iball, etc are some of the low end ups available
Edit: I meant 600 watts not va. if i have to mention it in va, it is 850 va or above. the reason for this is that to output 500 watts, your smps needs 600 watts as the input (under peak load) of course, if you do not have heavy graphics cards then 650 va will also do