I know there's alway eBay, but I'm a little reluctant to just plop down a bid and have it be over with. Having a game I'm constantly looking for gives me cause to head into every game shop I run across and dig through the piles of old carts. I even have an excuse to drive by garage sales slowly if the thought pops in my head.
That being said, I do have to admit that I looked up the price of a cart on eBay and every auction was up for under $10 at the time of writing, with a PAL cart being the closest at $9.50. With the announcement of a new title in the NBA Jam series, I'd think there'd be more interest in the auctions up, but only the one ending soonest had a bid on it. At $5 (after shipping), maybe I'd do well to cast a bid on the cart currently set at a $2 bid.
I'd much rather pick up the game in person, finding it in a small mom and pop store, especially now that Gamestop no longer carries Nintendo 64 items in any way. It sounds stupid, but it's a much better story to tell about the cart some years later after looking through my collection again.