How difficult are they to find? Very easy - think of any large corporation and they will have a graduate scheme. There are also websites like Gradcracker for STEM-related grad schemes and your uni careers service will help. For accountancy specifically, Big 4 and the ones outside e.g. Grant Thornton, BDO, RSM have them.
How difficult is it to get one? Quite difficult, but you can prepare for it. Practice psychometric tests, read up on assessment centres, and practice your interview skills.