Where do I go to continue my story plot/prompt?

I'm not sure what you're asking. If you want some ideas about how to proceed with this premise you ought to have asked "Where do I go with this?" and not "Where do I go to?" That phrasing doesn't work in the same context. Perhaps you're asking where you might be able to find more detailed prompts. The answer to that would be: Practically anywhere as they are all over the internet. 

If you're asking for help with how to proceed with the story, then it would seem that writing prompts are not helpful to you at all. You already have a premise, you just don't know what to do with it. At this point, you're not looking for inspiration, you're looking for someone to do it for you, which raises the question of why you might be interested in bothering to begin with. 

Here's a little trick you can use that might be of some help to you: Who? What? When? Where? Why? How? Which? Ask yourself questions about the story you'd like to write and answer them. Who? A couple of witches. What about them? They're rubbish at witchcraft. When? (Present day? 17th century Massachusetts?) Where? (A big city, a small town, etc.) Why did they become witches/why aren't they any good at it? They are both descended from a long line of witches, they just thought it would be interesting, they wanted to learn witchcraft for a specific purpose/they try to be good at it, but it's just not working out, the fact that they're awful is part of the plot - it adds humour to the story, etc. 

I could sit down right now and have a readable story based on this premise in 30 minutes. Why can't you?   

You have been 'racking your brains' - that is putting pressure on them to get an answer - like a victim being tortured on the rack. If you want to be a writer you need to get these things right.

Where you go is to see what calls to you.  Only you can write your story.  No one else will know what feels potent to you.  Your question is a good one, of course.  Most, if not all, writers have come to this very same point.  They have a great idea, but then it goes nowhere.  Here are some things you can try.

1) Do a free write.  That is, write as fast as you can, just letting the words pour out of you, about this general subject.  Don't censor anything that comes up.  Free writing helps the unconscious loosen up a little.  It could be an incredible idea will come up.

2) Research online about witches.  Perhaps you'll find something that triggers some great ideas of your own.

3) Put the idea aside. . . like write it down on a piece of paper and hide it somewhere.  Then find it in a month or even three months.  Sometimes what happens with this sort of technique is that you think about the plot without realizing it, and then you are ready to write and write and write.

4) Give up.  Sorry.  The truth is that not every idea, even every great idea, is usable.  It could very well be that you can't write this story.  

Remember, please, that the idea is yours and no one else's.  It may feel like someone is helping you if they direct your writing for you, but it actually means that the story is no longer yours.  You lose your power as a writer instead of building on it.  And, in the long run, that doesn't help at all.

Good luck with your writing!