If you're in the UK, you can take whatever grade exam you're good enough to take, whenever you like. I've never done piano grades, but I only did 3, 5, 6 and 8 on flute.
You can download the current syllabus for grade exam material from the exam boards' web sites, ABRSM being the main one in the UK although there are others that are getting more established, and either get one of the books of selections from each exam listing, or find the pieces separately yourself. Any good music shop will stock them, and I'm sure most will be floating about online in one form or another. ebay can be worth a look for a cheaper alternative to the highstreet. And second-hand shops, of course - you don't need a brand new edition, you just need an accurate one.
Unless you plan to either have a serious academic musical career (ie, you'll want to pass the higher grade exams as a means to get into college or university), or you've decided you want to do them as a sort of marker system for your progress - don't get too hung up on grades. If you're playing for pleasure, play music that gives you pleasure and don't worry if it's not on the grade lists or you don't know what grade it is. Good music has precious little to do with grades.