Activity measurements of alpha-particle sources with significant thicknesses require corrections for the self-absorption of alpha particles in the source. The dependence of the self-absorption coefficient on source mass varies according to the chemical composition of the substrate, and is often obtained empirically by measuring a great number of sources with known activity and different thicknesses. In the present work, we use Monte Carlo simulation for this task, applying the computer code SRIM, developed to simulate the transport of ions in matter, to the determination of the self-absorption coefficients for alpha-particle sources measured in detectors with a counting geometry. In particular, the coefficients were calculated for sources of uranium dioxide containing 235U, with a wide range of source thicknesses: from 1.0 to 10 mg/cm2 (thin sources, with thicknesses smaller than the range of alpha particles) and from 11 to 30 mg/cm2 (to cover self-absorption in thick sources). The behaviour obtained for the dependence of self-absorption on thickness for these two thickness regions was compared with a simple theoretical model. The simulation results were in good agreement with the model, although there were some discrepancies for thin sources, which can be explained by taking into consideration the scattering of alpha particles in the source. The present work shows the utility of using the Monte Carlo code SRIM to evaluate the self-absorption corrections needed in the measurement of alpha-particle sources.