  We consider scenarios in which the lightest sparticle (LSP) is the gravitino
and the next-to-lightest sparticle (NLSP) is a metastable stau. We examine the
production of stau pairs in e^{+}e^{-} annihilation at ILC and CLIC energies.
In addition to three minimal supergravity (mSUGRA) benchmark scenarios proposed
previously, we consider a new high-mass scenario in which effects catalyzed by
stau bound states yield abundances of {6,7}Li that fit the astrophysical data
better than standard Big-Bang nucleosynthesis. This scenario could be probed
only at CLIC energies. In each scenario, we show how the stau mixing angle may
be determined from measurements of the total stau pair-production cross
sections with polarized beams, and of the tau polarization in stau decays.
Using realistic ILC and CLIC luminosity spectra, we find for each scenario the
centre-of-mass energy that maximizes the number of staus with \beta \gamma <
0.4, that may be trapped in a generic detector. The dominant sources of such
slow-moving staus are generically the pair production and cascade decays of
heavier sparticles with higher thresholds, and the optimal centre-of-mass
energy is typically considerably beyond 2 m_{\tilde\tau_1}.
