If you want to create yourself cheap small-scale spaceships, you have two options available: Buy or find free papercraft starships, or scratchbuild them. This article shows you basics of scratchbuilding starships.
Materials and rules for scratchbuilding
1. Keep your eyes open; use anything, salvage things
That's right. You have potential starship pieces everywhere - or pieces of machinery, fortifications, industrial complexes - quite about anything. If you start to look for model pieces everywhere, you'll soon start finding them. When you start to get feel of scratchbuilding, you'll also find yourself asking salespersons weird questions. You may find yourself in a button-shop browsing through all of their stores, or asking the seller for buttons of very specific shape and material, no matter what the color is.
Button stores are one excellent option to collect resources for scratchbuilding. Various hobby stores are another great suggetion (ones that have cardboard, pieces of wood to make jewlery etc.), and one place you should also check is an electronic component store.
In addition, if you are throwing an electronic or mechanical device away, try to open it and see if it has any interesting -looking pieces inside it that could be salvaged and stored for later use. You WILL want a large collection of various pieces to be able to scratchbuild stuff!
2. Cardboard as base material
My single most important material in scratchbuilding is cardboard. Before starting to work with it, I suggest you read my article about Basics of cardboard modelling. Cardboard is horrible for round shapes, though - you may use it for rings, but if it should be round in two directions, you better find something else. As a good example of models consisting mainly of cardboard, see my Centauri warships.
3. Various materials
Depending on the model you are trying to create, you are going to need various materials you can create models or their pieces from. For example, in addition to cardboard these Hyperion cruisers use Balsa wood, wire and pieces of plastic. These Minbari Sharlin's couldn't have been made without modelling clay that hardens in owen.
An example of a scratchbuilding project
The photos below show the building process of a tanker-freighter. You can see that it's almost fully wood and cardboard; A few small pieces of plastic complete the model; A piece of wire keeps the round pieces of wood together.
And here's a few various pieces placed next to each other, showing you something that looks pretty much a space ship...
Want to see more scratchbuilding? This site has several pages showing you scratchbuilt models of various types, and many of them also show you the creation process.