There are five issues of Tumblers Today Magazine in Fallout 4. Each time you read one of them, the mini-game to lockpicking is easier, because the scope that you have with your hairpin / bobby pin increased. So if you collect all five issues, even heavy locks are easier to open. So it is best to get them as early as possible. Here you can see the locations of all five Tumblers Today Magazines:
The first issue can be found in Malden Center near the Vaults 75 in the Northeast. You can pick up the magazine in an orange Wagon, which is located in a large hall with many raiders.
You can find another issue of the Tumblers Today Magazine series in the Poseidon Energy Turbine 18-F. You can find it in the northwest of Bunker Hill. In a cottage there is the booklet.
Also at Bunker Hill, but to the east of it, you'll find the East City Downs, where Raider organizes races with robots. You will find the issue in the cabin of the stadium announcer.
Go to the West Roxbury Station, which is located south of Diamond City. Hold on the left and go down the stairs to the platform. There you have the pressing buttons to move trains, there you uncover the path to the other platform and the magazine.
For the last issue you have to go down to the stinking and monster-infested Fens Street Sewer. Your entry point is located directly in the northwest of Diamond City. Enter through the gully and shoot you through the critters until you discover the booklet.