Buy some pegboard/perforated hardboard (thin board with holes in it) from a timber merchant. If you are local to Bristol we bought ours from Robbins Timber in Ashton Vale - they were extremely pleasant, cut it to size and carried it to the car when I turned up to pick it up with two children. I bought the pipes and the blue funnels from the Children's Scrapstore (just about my favourite place in Bristol). Everything else I had at home - you can just use whatever bottles/tubs you have in your recycling. I made some holes in the bottom of an ice cream tub with sharp scissors to create a sprinkler effect, and cut up two bottles to go at the end of the other pipes. I fastened everything to the pegboard with cable ties. I balance it on an underbed storage box and have tied it to the back of a garden chair to make it a bit sturdier. This was pretty quick to make considering the amount of use I think it will get over the summer, and it's going to be easy to upgrade and change round in the future too.
See it in action:
My daughter has been watching lots of Frozen Playdoh demo videos on YouTube recently (yes, quite niche but they do exist) so she was really keen for me to make a video of her playing with the water wall, in the same style. Hopefully this video is less grating than the Frozen Playdoh ones - it definitely shows her enthusiasm!