You simply need to search for tongue twister exercise on the search engine and you will find lots of blogs offering free sentences for you to practice. It is encouraged that you read out the tongue twister sentences aloud in order to improve your pronunciation.
The S ones are the ones that get me. Try this one: She sells seashells by the seashore.
The shells she sells are surely seashells.
So if she sells shells on the seashore,
I'm sure she sells seashore shells.
This one was always a favorite of mine when I was younger: How much wood would a woodchuck chuck
If a woodchuck could chuck wood?
He would chuck, he would, as much as he could,
And chuck as much as a woodchuck would
If a woodchuck could chuck wood.
I have never heard of that one before Rafiki!
What about Betty bought a bit of butter.
But the butter Betty bought was bitter.
so Betty bought a better butter,
and it was better than the butter Betty bought before.