Are there any lightweight baking containers that can be used in both microwave and oven. I have bought prepared meals in these but have never seen them for sale. I do not want glass.
The material you'd want is silicone.
For example, there are silicone baking molds. They are often used in professional kitchen settings, but I'm sure you find find some online. Here's an example.
There are probably other kinds of silicone containers that can handle oven well, and they can definitely handle microwave.
You could go with a lightweight tempered glass product like vitrelle (aka Corelle). They make bakeware that is lightweight like their dinner plates. You would get all the benefits of traditional glass, without the weight.
The other thing you might be thinking of is the plastic carryout containers that are oven safe up to 400 degrees (or whatever its rated for). Search "Oven Ready Plastic Bakeware" on the web.