Technically the tie has a light purple vertical stripe on it as well, so if you wanted to be super accurate you could try sewing a piece of thin ribbon down a black tie. You can probably find every shade of ribbon under the sun in a craft store.
Also to be technical, I've heard people say the jacket is "periwinkle" but the precisely exact shade shouldn't matter as long as both jackets are the same. If they're not it will really stand out with the two of you next to each other. (I only mention it because I've seen Host Clubs who all have different jackets and it really takes away the School Uniform effect)
You can either buy an Ouran jacket, or look for a similar jacket in a store and buy the Ouran patch off ebay or from a cosplay shop. They're everywhere. The jacket may be hard to find in a store though, depending on how accurate you want it to be, so buying the jacket online might be your best bet.
I'm not the one to give advice on cosplay stores though, sorry, because I make my cosplays. (doesn't want to sound prissy, just saying, I don't know which ones are good)
With the white shirt and black pants, those should be easy.
Call me Ari if you like
Hirasawa Yui [Uniform] (K-On!) - 60%
Hirasawa Yui [Don't Say Lazy] (K-On!) - Planning
Miroku (InuYasha) - 95%
Imp Midna (Twilight Princess) - 40%
Cons: Animaritime 2012 and Hal-con 2012