When I got started with watercolours many years ago I tried them out on photocopy paper and it was VERY bad, the whole thing went see-through and wet and floppy in seconds, the paint looked terrible and it just fell apart, literally. Next I tried painting on thick card - again, a disaster. Why? Because regular paper/normal thick card/canvas paper etc. cannot handle having heaps of water on them, they buckle, fall apart, or sometimes the water/paint just run straight off!
So, what do I recommend? Buy some proper watercolour paper! It doesn’t have to cost the earth, but some key things to look for:
Buy WATERCOLOUR paper, nothing else
Paper weight - I buy 300GSM (this means it’s really thick and can handle more water)
If you are painting large pieces with lots of water (e.g. a landscape) you may want to stretch your paper, or use a 'watercolour block' - which means your paper won't buckle.
Tip 1: You could even try a Moleskine watercolour notebook - the paper weight is slightly less than what I suggested above, BUT, all your sheets of paper are in one spot, not sheets of paper scattered throughout the house! Plus, you can flip through it in years to come and see how your artwork has progressed. I have several and love them all! They cost a bit more but they have zillions of pages, so it’s actually really good value for money.
Tip 2: Do you have a guillotine/paper cutter? If so, you could try buying A3 pads of watercolour paper and trimming them down! This is a cost effective option.
Another great resource: your local art shop! Go in and ask for their advice, tell them your budget too and they can help you find something that is great quality within your price range.