Using ROM images from the net, or even from a copy of a game you own, is illegal.
Here is Nintendo's official take on it
If you want to be legal, you have to buy the original system or a licensed re-issue and the original ROM.
As these games are out of circulation, and as such are practically relics by modern media standards, you've got to choose your battles.
Finding Them Cheap/Free:
If you want to be thrifty and save a buck, you're going to have to hunt around and do the leg work. These items can be found at many vintage/thrift/flea market locations... often for 99¢ So you could scour the wastelands and possibly score one. It won't be MIMB... But it'll play your games.
If you REALLY love the game just buy it. No price can be put on love!
Own it and treasure it. Archive it.
Take pictures of it with you on vacation and frame them.
And check Craigslist. People often GIVE AWAY old stuff like this.
You are essentially collecting art and design from the past... just with the added bonus of light, sound and interactivity. Sometimes really amazing pieces of the historical past of video games fetch VERY high prices. If that is the case, just owning it can be a fight. Be prepared.
Places to Buy Old Games:
There are a lot of places that ship and have online inventory or are only a phone call away.
I can personally recommend eStarland - They have long been a leader in online retro and import game support.
8 bit and up / 212-674-0201 - Very cool owner, and great shop.
Video Games NY / (212) 539-1039 - Has a huge selection
You can also purchase a reissued console that plays both NES and SNES (and Sega's SMS/Genesis - score!) cartridges for about $30
CNES/SNES/Genesis FC3 Plus Gaming System
These systems support the original cartridges. And are also legal.
In short, collecting vintage via the internet isn't always cheap.
To save money you're can try questing around the country side of suburbia. MILLIONS of SNES systems are always around. Plus it's always good to get out of the house, off the net and meet your neighbors!
But, unless you own a time machine, be prepared to be the high bidder or forever be incomplete in your collection if you're after a hot or controversial title.