https://www.amazon.co.uk/Japanese-Kanji-Flash-Cards-Kit/dp/4805311746
Google is easy to use.
There might be some apps on Android/iOS but what works best for me are physical flashcards. I bring them everywhere I go and usually when I sit down, I take them out and check few of them throughout the day. I use this set from Amazon, but you can also use Anki decks if don't want to spend money https://www.amazon.co.uk/Japanese-Kanji-Flash-Cards-Kit/dp/4805311746/ref=sr_1_1?crid=3277GEPZ5J38L&keywords=kanji+flashcards&qid=1555267983&s=gateway&sprefix=kanji+flashcards+3%2Caps%2C148&sr=8-1