Could you make one? I'm nearly always disappointed with anything I pay out cash for for the garden, could be bigger, could be that, could be this. I find garden woodwork a lot of fun though I don't have much skill at it, but then one doesn't have to be mm perfect with it either. I just put up a bench in my potting shed, got some help from my son and it's a perfect fit, perfect height etc. Cold frame is next on my 'to do list'.
I agree, if you or family/friends have any basic DIY skills, they are very easy to make.Mine is made from decking planks with a sheet of twinwall polycarbonate used as a sloped hinged lid and was cheaper and is stronger than anything commercially available that I have seen. Here's a previous thread on this topic:
Years ago I learned that I did not have the skills or energy or patience for many of the day garden jobs that some on this forum excell at.
unfortunatley it does not stop me accumulating old window frames, shower doors and broken wooden beds in the mistaken belief that they will come in useful.
having said that I have created a hedgehog house out of an old urinal and a hot bed from an old leaky domestic water tank.
I think the old window frames is a good route for a cold frame. I know a joiner who might be able to supply me some. The lid has to be the hard bit, so if one can start with a window frame for the lid and build the box around it, half the job is done already.