Do you have a pull-up bar or access to one (ie. in a park or a playground; you can hang off most things)? If not, you can invest in one of those doorframe pull-up bars - they cost about 20-30 dollars usually, which is not prohibitively expensive. If you do decide to head out to a playground, look for a place where you can do dips (you'll need two parallel bars at shoulder width and hip-height, approximately).
The reason I suggest these is that you need to do these "hard", compound exercises if you want to gain muscle; push-ups will only get you so far (and that's not very far indeed). It is also relatively easy to progressively load them further if required - you can use a backpack and fill it with weight. These two exercises will work most of your upper-body and require minimal equipment. If you are concerned your abs aren't getting worked out, you can also invest in an ab-wheel, which costs about 10-20 bucks usually and is also a very good tool.
For bodyweight exercises in general, the way to make the exercise more challenging is to decrease the leverage you have - eg. for pushups you can raise your legs on something, which'll stress your shoulders some more (and in general, you'll be lifting a higher weight) or you can try to push your arms more forward (ie. don't keep them bellow your shoulders but further up towards - or even beyond - your head), this will work your abs and lower back muscles more and is harder.
Still, to answer your actual question, if you are determined to "make" your own weights, the best solution is probably bags filled with sand. Take some of those small bags which you use to pack stuff in the fridge, fill them with eg. 1 kg of sand (don't forget to double or triple wrap them, they can break and sand everywhere is not comfortable). You can then put as many as you need in a backpack or a bigger bag/container and easily calibrate the weight you need. Note that it's actually non-trivial to figure out how to add weight to bodyweight exercises (and doing something intended for dumbbells with homemade weights, you'll outgrow fairly quickly). If at all possible, I would try to assure access to at least a pull-up bar and exercise with it.