You don't need the CSU/DSU to practise for Frame Relay. Just connect the routers back-to-back, but be sure you have both DTE and DCE part of cable connected in one piece. Then on the router to which you will connect the DCE part use "clock rate" command to simulate clocking function of real CSU/DSU.
Peter Mesjar
CCNA, A+ certified
pmesjar@centrum.sk