Where can you get a Tanooki Suit in Super Mario Brothers 3

There are 3 Warp Whistles in Super Mario Bros. 3.

The first warp whistle is gotten in stage 1-3 by hopping on the white block, then pressing down for several seconds to drop into the background, then running all the way to the end of the stage. Be aware that you need to hurry as this effect does wear off.

The second warp whistle is in World 1 Fortress. You need the Raccoon Leaf to get this one... At the end of the first area, instead of going through the door, stomp the dry bones then run from the far right side to the edge to build up speed so you can fly. Fly up and to the right. You'll land on a hidden ledge above the wall. Run to the right and then press up to enter a hidden door with a chest with the warp whistle in it. Note: This completes the fortress, but doesn't remove its door on the map.

The third warp whistle is on the World 2 overworld. You need to get a Hammer by defeating one of the Hammer Bros. sets wandering around the map. Once you have it, go near the rock in the upper-right corner of the map and use the Hammer. It will break the block. Behind the block is a wandering Hammer Bros (Fire Bros. specifically). Beat them to get the third whistle.

However, you only need two whistles to get to World 8...

There were two hints that the warp whistle was there, and both of them were contained in letters from Princess Peach.

Between worlds 3 and 4 you get this message:

The White Block contains magic powers that will enable you to defeat your enemies.

And between worlds 6 and 7 you get this one:

I am well. Please retrieve the Magic Whistle hidden in the darkness at the end of the third world

Although it's not a map and a sign, both hints together are enough to point the way to this particular whistle at 1-3. It looks like a basic way to reward a replay as you made it through the game. There are even less clues for the other whistles; one other letter makes reference to "east of the sand dunes" for the desert world whistle, and for the first fortress whistle, there's nothing to hint at it other than Super Mario Bros hid a warp zone in exactly the same way (plus as mentioned by Ryan, it was practically advertised in a movie).

Otherwise, intentional vagueness and cryptically hidden random-access secrets (like white mushroom house, and pirate ships full of money) was a good way to sell video game guides in the early Nintendo days.