where is the best place to park to visit adelaide botanical gardens?

The botanical gardens are very well served by public transport. If you have to take a car, park outside the city and get a bus to North Terrace. North Terrace is a delightful road with not only the botanical gardens but the university, the library, the museum and a number of small museums and other points of interest.

If you insist on driving all the way, there are plenty of places to park. Check Google maps https://www.google.com/maps/@-34.9186655,138.60652...