your product/training supplier will supply them. if you do a google search for nail trainer it will bring up various companies that sell them as well...ie: essential nails, nsi, sweet squared, even ebay!!
i definately wouldnt soak them off, as they arent designed for this purpose. i think its nice to keep them, that way its good to look back at them and see how much you've progressed over time. plus when youve qualified, it comes in handy for practising new designs, techniques or products. ive got a folder with loads of nails in with all my nail art designs on that i let my customers look through xx