There is an inexpensive method to test for DO. However it requires some time and simple titration using drops of titrate chemical. Titrating a smaller volume using several drops of titrant gives fairly good results. However this as a laboratory method can be very accurate but it is no way near as quick as using a testing meter. Meters are expensive but save time. Time is money to some people.
The basic method is called Winkler Method. There are very simple test kits up to the more complex standard laboratory technique that can give very accurate results. I learned DO testing basics in college using the standard Winkler Method. For a simple test kit, I would go to a reputable pet shop and ask the aquarium person to see the DO test kits. Simple test kits would also be available on Amazon.
See if this link works for you.
https://www.amazon.com/s?k=dissolved+oxy...b_sb_ss_i_3_17 Or just go to Amazon and search for dissolved oxygen test kit.
One of the important items in the Winkler Method is to collect the water with minimal exposure to air. Air exposure allows air's oxygen to dissolve into the water sample to be tested thus giving a slightly biased result. If real accurate results are needed there are special methods to collect the sample that minimally exposes the water to atmospheric air. Just pouring the water from collection jar to the testing jar adds a little DO.
