It is usually the reduction of the elasticity of seals that causes them to fail with age/use and not shrinkage. I suggest that you take the part to a well established parts shop and see if there is an alternative seal of the same dimensions. Or you could compare the measured dimensions of your seal for seals from other cookers online, and take punt. Some of the Tower models seem to be the same as Fagor models and with those Fagor seals fit, for example.
Phil., August 2013