For many years i have suffered really heavy periods as a result of this?

Any of the well known brand of waterproof mattress covers will do the job and I would suggest a Terry towelling kind. In the UK I would use John Lewis, Marks & Spencers or The White Company etc. However, have you ever thought of doing something about these heavy periods. No woman should have to put up with that month after month. Talk to your GP, there are quite easy ways to try and help this problem. A mirena coil might help for a start but check out there's no problem first, like a fibroid or such like. Enjoy the new mattress.