You can contact the CIS Helpline at HMRC by calling +44 300 200 3210.
You will need you UTR number and National Insurance number, which you can find on your profile.
You should explain that you are a CIS subcontractor, and that you have had difficulty getting hold of some of your statements, due to your contractor(s) not providing them.
You should also mention that you have tried contacting your contractor(s), and done everything in your means to recover the information, but have been unsuccessful.
You will need to know what tax year you need the statements for (this will be something like 2016/17 or 2017/18).
Remember that it is your responsibility to have made sure you received this information, and also to have kept it safe. They may not be to be willing to help you at first, so have a bit of patience and be nice, they are doing you a big favour!
Once they agree to send you the information, you should receive it within 2 weeks. Once you have it, you can login and add it to your account.