Life Hearts are abundant in all levels except the first, with the exception of boss battles.
Here's a run to get a life boost for hard levels: Start at the Transit Station in Main Street . Instead of jumping out the window over the truck, run down the little passage on Vince's left, hop the gap where a set of skull pages were collected, jump & hover down to the first Heart on top of the underground broken pipe area. Then proceed up the steps, out onto the balcony and go to the right, jump, then hover the balconies to the awnings for Hearts 2 and 3 as Vince'll following the street back to the door that leads to the Docks. (Catching onto Propellers wastes precious time.) Follow this path finally to Heart 4 at the dead end of the street, (over the door), jump down & enter the passage that leads back to the Docks. Run down the high wooden path where the Piggy Bank of Doom was defeated and hover down to Heart 5 on the post out in the water. Jump back past the Bubba Gator on the dock & up the hill to the left where Heart 6 is under the wood walkway guarded by a couple of Pit frogs. Now back to follow the wooden path back up to the Trolley grabbing the final Heart on the way. Take the Transit Main Street, rinse and repeat until Vince has ninety nine lives. Don't waste time fighting monsters, Just run past them.