The cheapest way is to look in 2x6 or 2x8 lumber or similar and pick out the best you can find, then rip pieces out, excluding knots, scarf together as needed to get to clear 12' lengths.

Otherwise, the cheapest clear lumber I have found in stock is called "select" where I shop, comes in 5/4" thick, an 8"x12' would be $30-$40.

A third option others have used is to have "clear" lumber special ordered at a lumber yard.

   ...and, for strength as well as appearance, the top and bottom layer or two are the critical ones. Fairly large knots in the middle layers have almost no strength effect.

Also note that straight grain in those outer two layers is just as important as "clear" or knot free wood. Grain that runs out in less than a scarf length (2.5" in your 1/4" strip of wood, say) is just as bad as a knot. 

Thanks for the input.  Hadn't thought about scarfing, I think that's the way to go.  And good point on grain direction in the outer two layers.  Makes a lot of sense.  Off to the garage!     

I'm not sure that nemochad was suggesting scarf joints, although that may very well be a viable solution.  I think he was suggesting that the geometry of a scarf joint is the minimum requirement for the grain pattern so as not to create a weak spot.  Scarf joints usually have at least an 8:1 ratio, so on my PM, a scarf joint on 1/4" plywood will be 2" long.