Are there any places that I could give away pet goldfish?

Our family went to a summer festival about 3 months ago and did the good ole’ 金魚すくい. We brought 3 cute goldfish home and we set up a 20 gallon (approx. 90 liter) aquarium for them. I did some fishkeeping when I was younger, but mostly with small tropical fish like tetras and gouramis. Two of the goldfish are growing very quickly. From 1.5 cm, they have grown to more than 4 cm in a short amount of time. I anticipate they will double in size within the year, which will lead to a lack of swimming space, etc. so I want to ask ahead. I did some reading on Japanese goldfish blogs, and it seems the species is called 和金. They are a very hardy species, but it seems they need a much larger aquarium to grow and enjoy their lives. I was wondering if there was anyone who has the experience of giving away pet fish and if they could give me some tips on how to proceed with this dilemma. Thanks!