What's a catechism?
Universal Catechism
If you aren't sure what you're looking for, this is probably what you want: Catechism of the Catholic Church
A handy table of contents with paragraph numbers for the universal catechism.
National Catechism
http://www.usccb.org/beliefs-and-teachings/what-we-believe/catechism/us-catholic-catechism-for-adults/
Not recommended.
Catechism for children, aka YOUCAT
These are very commonly used.
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https://www.youcat.org/products/youcat-for-kids/
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https://www.youcat.org/products/youcat
Baltimore Catechism
https://en.wikipedia.org/wiki/Baltimore_Catechism
No 4, an explanation of the Baltimore Catechism for catechists