I'd go to garage sales and look for one but other than that there's always E-bay and stores that sell old games like EB Store and Game Stop. But the NES is getting harder and harder to find so the sooner you start looking the better.
I may get another NES soon so I may have one to sell later if I feel like it. For 50 bucks! J/K But if I find one I'll be sure to buy it, they can easily go for $15 , but some won't let them go unless you hand over at least $45. People are starting to catch on to their value.
I've seen them at flea markets, goodwill, yard sales etc. Your best bet would probably be to start asking friends and co-workers if they still have theirs and if they'd be willing to sell it. Everyone had one and many people still have it stored away somewhere.