How's your scales and arpeggios? Have you played some Etudes by Czerny, Burgmuller, Heller etc?
This piece is fairly advanced. Why don't you give us an idea of where your technique and current repertoire is at so you can get a more informed advice on how you might want to proceed.I'll add on to what lelle said by suggesting you're played at least a few pieces with 4 against 3/3 against 4 polyrhythm.
Well, if you really want an etude, you can play scales in 4:3 polyrhythms. Just C major scales up and down in a 4:3 polyrhythm should help.
There's a beautiful Chopin etude which deals with 4:3 polyrhythms in a slower tempo than Fantaisie Impromptu. In the etude it's the other way around - the left hand has 4 notes and the right hand has 3. It might offer you some valuable experience before tackling the Impromptu.
There are Etudes by Roslavets and Ligeti that deal with polyrhythms (as for Ligeti, not all of them, maybe Desordre (please pardon if my spelling is wrong) or Vertige, I dont know). And practise the ending climax of Chopin's last ballade, it has 9:6 polyrhythms. Hope this helps.
Where can I get them
I think you have to purchase them in a store or online since they're still under copyright (more modern composers).
Topic: I love to learn Fantaisie-Impromptu 