It depends on the authors. Some of them don't release their code at all. Usually you should find on their website some information and the standard procedure is to cite them.
if you need a fair comparison, i recommend you to contact directly with the authors of such works. They would facilitate the articles in question. Regarding the matlab code, as Aniello points, is not as easy to obtain. Nevertheless, authors should explain the procedure to get the results obtained in their publications and this may be a helpful tool to achieve your aims.