Whenever I need turbo on my famicom, I make use of this little device: the Famicom Multi AX-1.
It is a bit tricky before you know how to use it. The trick is that you actually use the second player controller to play as the first player controller with turbo.
The settings are a bit confusing at first. There is one option that allows you to control both controllers at the same time, should you want to play co-op Twinbee with yourself, for example.
To use it for the turbo-play it should be set in the settings above. Then take your two-player controller, and start playing Gradius, 1942 or whatever game you need turbo for. There is a dial that sets the speed of the turbo, which is useful in shooters when you can only fire limited amount of shots. If your two-player controller doesn't have a pause button, keep the one-player controller nearby for when you need to pause.