The only way you will find a LA to practice on is:
A good quilting buddy who has one willing to let you practice on it
A quilt shop that offers rental time on their LA that will make you take a class on it before allowing you to rent time
A quilt show that has loads of LA vendors there to let you try out the machine, although I wouldn't necessarily call that practice
A longarm dealer willing to let you rent time on a machine before purchasing, again you will be required to take some sort of training class.
Even used LA machines aren't cheap. They tend to not lose their value so a LA set up, even used will run anywhere from a few thousand to over $10K.
PS welcome to the board!