Building a robot can be expensive, so finding free robot supplies around you is usually a good idea.
Hopefully, it can be really easy to find a lot of that supply. The few hard things to get are the microcontroller and some other specific sensors or components. And as you get more experienced, you'll learn how to find some these too, plus you'll be able to scavenge robot parts from your older projects.
Here I tell you how to find a lot of pieces for the frame, the wheels, and a few components like motors all for free or less than a dollar.
I have a supermarket 1 min away, and they get rid of their cardboards around 11am every morning. Of course, if you come between 10 and 11am, you have unlimited supplie of cardboard.
Choose some good sturdy cardboard, that is hard to fold. They are good for the frame, and you can make wheels out of them too.
Depending where you live, they may also have some light wood. It's not the most resistant but it's good enough for small robots in my opinion.
I don't if you can those from the supermarket, but you can get hangers for free. If you need some good solid wires, go to the nearest shopping street and ask some clothe shops if they have some hangers they can spare. My brother did it when he moved in his new apartment, got way more than he asked for...
You can also get wheels from caps of jam pots at home, for example.
If I want those wheels, I'd better get to work!
Anything with electronics inside that you no longer use is a good candidate for scavenging. Plus, you may learn something about electronics design, or see that it is very easy to repair it.
Old computers, especially hard-drives, are a good place to find brusless motors, if you know how to control them. They have also all kind of electronic robot supplies. Robots are mainly computers on wheel, aren't they?
A tupperware-like container is perfect to hold electronics and become a frame to your robot. In Japan, I get them for 100 Yen each (a little more than a dollar) in the 100Yen Shops. Try the $ Stores in the US and I hope you can find some there too.
In these same 100Yen Shops, I can find cheap toys to use as frames, or just take away the wheels. Of course, if you still have your kid toys collection, you can find it a second use...
In my opinion, find free robot supplies is all about the mindset.
Once you believe that anything round is a wheel, anything plane can be part of the frame, and any electronics can be reused, you will find your robots are a lot cheaper.
So try out with anything you can, test what works for you. As you learn more about electronics, you'll find more useful components at home, too.
Good luck building your robots on the cheap.
Other pages that may interest you:
A list of useful robotic sensors for your projects. You'll also learn some ideas on how to use them for your projects. Sometimes, you just need to know what sensors exist to inspire new projects.
Choose the right robot motors. This basic guide present the 3 basic types of motors, how to control them and some shopping recommandations.
How to choose your first robot? Need help sorting the options for your first robot? This is the best guide to choose it.