If you are working with Rayleigh fading channel with coefficient h that is Gaussian with zero mean and variance 1, then the instantaneous SNR is exponentially distributed with mean equals to the average SNR, Eb/N0. What you can do is for each Eb/N0 generate N exponentially distributed RVs that represent the instantaneous SNR (in MATLAB the command is exprnd(Eb/N0)) and see how many times the instantaneous SNR is below a certain threshold. The ratio between this and N will the outage probability for this Eb/N0. Repeat for all Eb/N0 values to get the outage probability for all average SNRs.