Why does Tom want to get a couple of case knives?

Tom wants to get a couple of case knives because he wants to use them to dig out the foundation from under the cabin (where Jim is being held). While Huck prefers to just steal the keys to the cabin, Tom thinks "there’s more honor in getting him out through a lot of difficulties and dangers." 

In other words, Tom wants to carry out the kind of great escape he has read about in novels....

Tom wants to get a couple of case knives because he wants to use them to dig out the foundation from under the cabin (where Jim is being held). While Huck prefers to just steal the keys to the cabin, Tom thinks "there’s more honor in getting him out through a lot of difficulties and dangers." 

In other words, Tom wants to carry out the kind of great escape he has read about in novels. To Tom, an admirable escape involves lots of complications and dangers. When Huck suggests that they use picks and shovels to dig Jim out, Tom scoffs at the idea. He tells Huck that nobody in the great action novels actually digs out criminals with big tools. 

Instead, those heroes always use case knives. Tom argues that the men generally dig through solid rock, rather than dirt. He also proclaims that the process of digging out Jim should take weeks and weeks. Huck puzzles over Tom's plan and concludes that Tom is being foolish. However, Tom tells Huck about the prisoner who ended up in China after spending thirty-seven years digging his way out of prison. 

Of course, Huck is still doubtful after hearing Tom's story. So, Tom decides to compromise. He tells Huck that they will only need to pretend that they spent thirty-seven years digging Jim out with their case knives. Tom's answer pleases Huck, and the latter decides that he will look for a couple of case knives.