Tests are released on the government website at 6am on Monday mornings
They are about 5 months in advance, so it's be for April or something, but there are tests released every week.
You can book cancellations - my test is December 5th. If I fail I will search with the cancellation app until I find another one. I've heard it's usually possible to get it within a couple of weeks this way.
You can also pay somebody else to do this for you, or some companies but up tests in advanced and sell them at a premium, so it's up to you. But what I will do is search for it myself, possibly with the premium version of a test cancellation app.
Good luck 👍