We report material optimization for a polarized electron source from strained GaAs:Be grown on an Inx0Ga1-x0P pseudosubstrate, i.e., a fully relaxed Inx0Ga1-x0P buffer layer on a GaP substrate, whereby the large lattice mismatch between the Inx0Ga1-x0P and the GaP was relieved by a linearly graded InxGa1-xP buffer layer. By changing the In composition x(0) in the Inx0Ga1-x0P, strained GaAs:Be active layers with various lattice mismatch were obtained, and the effect of growth conditions on the residual strain in GaAs as well as the strain-induced enhancement of electron-spin polarization were studied. The results show that the residual strain in the GaAs layer is very sensitive to the growth temperatures, and that by choosing the right growth conditions, the strain in GaAs can be maintained even when the film exceeds its theoretical critical layer thickness. Enhanced electron-spin polarization has been observed from samples with larger strain.