There are many levels where you can easily get 1-ups.
When I’m low on lives, I currently play world 1-1 to get about 5 or 6 lives per shot, but I think it’s not the most effective solution.
What is the fastest way to get lives?
This is my personal favourite way:
Go to level 3-2. Play the level until you get to the section with 4 bullet bills with a green warp pipe in the middle. Keep hopping on the bullets shot at you without touching the ground (hint: utilize shaking the Wii remote to stay in the air slightly longer). At first you will only gain points, but eventually every bullet bill you hit will get you a 1-up. It might take a while to get the hang of, but it’s worth it.
Here’s a video of the process.
You can also gain 1-ups quickly in level 2-3. At the very end of the stage, you have to run as far right as you can on any stair. Then, jump so that you land on the Koopa that is falling down the stairs towards you. After that, don’t press anything. This method is much easier (if you can time the jump properly) as you just have to let your character jump on the shell repeatedly. Here’s a video of this one.