You can certainly buy off the shelf PC coolant from many sources.
You can use distilled water with a biocide, again something you can buy off the shelf.
Most AIO are mixed metal solutions with a copper chill plate and an aluminum radiator. In which case you want a coolant with an corrosion inhibitor. Again, you will find many off the shelf marketed products.
AIOs generally only last about 5 years, so I wouldn't put too much effort and money into this.