where to find downloaded library of sbt?

All new SBT versions (after 0.7.x) by default put the downloaded JARS into the .ivy2 directory in your home directory.

If you are using Linux, this is usually /home/<username>/.ivy2/cache.

If you are using Windows, this is usually c:\Users\<username>\.ivy2\cache.

EDIT:

Here’s an example from one of my projects,
in which I define an SBT task that copies the dependencies into the target folder.
You can place this code into your project/Build.scala project definition file.
You should have something like this in your project definition file (more info at www.scala-sbt.org):

import sbt._
import Keys._
import Process._

object MyProjectBuild extends Build {

The following code copies all your libraries to a deploy/libz subdirectory,
by defining a deploy task that captures your program artifact and all its classpath dependencies:

val deployKey = TaskKey[Unit](
  "deploy",
  "Deploys the project in the `deploy` subdirectory."
)

val deployTask = deployKey <<= (artifactPath in (Compile, packageBin), dependencyClasspath in Compile) map {
  (artifact, classpath) =>
  val deploydir = new File("deploy")
  val libzdir = new File("deploy%slib".format(File.separator))

  // clean old subdirectory
  deploydir.delete()

  // create subdirectory structure
  deploydir.mkdir()
  libzdir.mkdir()

  // copy deps and artifacts
  val fullcp = classpath.map(_.data) :+ artifact
  def lastName(file: File) = if (file.isFile) file.getName else file.getParentFile.getParentFile.getParentFile.getName
  for (file <- fullcp) {
    println("Copying: " + file + "; lastName: " + lastName(file))
    if (file.isFile) IO.copyFile(file, (libzdir / lastName(file)).asFile);
    else IO.copyDirectory(file, (libzdir / lastName(file)))
  }
} dependsOn (packageBin in Compile)