Thank you so much this is incredibly helpful!I don't know of any question bank entirely dedicated to competency question, but TCLA offers an excellent question bank with 320+ questions, over three quarters of which come from actual interviews. It is split in various sections, and has one specifically listing a few dozen competency questions. You can take a look here.
As for my advice, I firstly wanted to link here a recent Guide to Competency Interviews I have written, which explains my step by step preparation approach and top tips. Besides this, I think to do well when asked a strength/situation-based question in an interview you should do the following:
- Pay a lot of attention to the exact strength, situation or competency you are asked about. Many candidates will have prepared for a very similar but not exactly identical question and will then have the temptation of making their pre-prepared answer fit. However, it usually comes off as a mixture of awkward and robotic.
- Take some time to think - if no idea immediately comes to mind as to how to answer it, ask for a minute to consider your response. Then try to mentally picture your CV and try to think which of your experience best fits the question. It can be very tempting to feel that you need to start talking immediately after you are asked the question, but if you have not thought of a way of doing it properly you will end up rambling and potentially even seeming incoherent. It is better to take the time to consider, and if you still can't find anything sensible to say, to own up to it.
- Be clear and structured yet conversational with your speech. A key skill you are tested on is an ability to communicate effectively, as the firm wants to see if you are the kind of person who they would one day be able to put in front of a client. Thus, more importantly than trying to show off impressive substantive points, you must make sure there is no ambiguity in the way you express your thoughts. However, avoid sounding to mechanical: try to nod, smile, to change the pace of your speech, and change your body language, your tone and attitude at various points. Finally, when possible, leave space for/invite commentary and back and forth conversation with the interviewer.