I have wooden window frames in my house which contain smallish sealed
double glazed units.

The units total dimensions are appox 49cm x 46cm x 1.7cm.

The units are held in place by four wooden strips.

All in all pretty cheap and nasty.

Is it possible to get replacement units and fit them myself. If so how
should I specify dimensions and where do I order them from.

I did phone one supplier but he said they need to be measured and fitted
by a professional which seem a bit strange.