buy one or make your own. I'd recomend 50' or 1/2" copper. I'd also highly recomend some sort of whirlpool as well. I do my whirlpool with a pump but if you don't have a pump you can simply stir it with a spoon. The whirlpool greatly increases the cooling effectiveness of the immersion chiller.
I do 10 gallon batches with a 50' 1/2" chiller and it works just fine. The whirlpool makes a big difference as well; assuming you have a pump it doesn't take much to get setup.
Used my 50 foot 1/2 inch chiller to take 11 gallons to 75ish in under 16 minutes last weekend. During the winter should take it to 65 or lower in 15 minutes due to the cooler groundwater.
Lifefermented does it really take you > 60 minutes to get to 70 ish degrees with your chiller or did I misread your post? That seems like a really long time.