Well there's always the quick and dirty options that don't act like plasticard but do have some of the properties and most of all are cheap. CD Jewel Cases are one option but you have to be careful because they can shatter and be deadly, so aren't ideal for cutting up. An alternative are old DVD cases. You can get just short of an A5 sheet of slightly textured plastic that can be cut, bent and carved for next to nothing.
But I think the cheapest most varied option is food trays. Chicken and meat trays are vac-formed with marvellous patterns on to help drain juices away. Of course one downside is durability and sometimes the thinnness of the material doesn't seem solid enough. There is a solution though which offers more answers than the questions...
When I produced my first building I mixed up a large batch of undercoat, my classic recipe:
- black interior house paint [because I have a large pot of it]
- black Early Learning Centre poster paint [for bulk and to offset the satin finish of the house paint with it's powderiness]
- Polyfilla, er filler [to make it have a bit of texture and bulk]
- PVA glue [bulk and make it more adhesive to the building]
- I used the chicken tray below to mix it all up in. This is the solution to making your plastic tray more sturdy. You could substitute all of the above for some plaster and when it sets the material will be strong enough to handle all knocks and scrapes.
Alternatively the use of PVA and acrylic paints meant that the excess paint I didn't use ended up casting pretty well. It's a bit lumpy on the back but nothing a quick sand couldn't improve and maybe one day I'll find a use for this, maybe as some treads for steps. So you can use the plastic tray as is, make it rigid with plaster or cast from the inside with a PVA solution. Regardless you have some nifty new textures to apply to building or use wholesale as floors etc.