If you already have a Linux system that you can dedicate to Asterisk, simply use the package manager in your operating system to install Asterisk, DAHDI, and libpri. Most modern Linux distributions such as Debian, Ubuntu, and Fedora have these packages in their repositories. Packages for Red Hat Enterprise Linux and CentOS are also available at http://packages.asterisk.org/ (see Asterisk Packages for instructions on use)
Linux distro maintained packages may be old, so watch out for that. There are no currently maintained official repositories for Asterisk packages.