I am looking to put on some aftermarket accessories on my 06 trx300ex. i have a few catalogs but many parts are listed for the 250ex and the 400ex. will these fit my quad?? i was looking at an airbox cover, nerf bars, and exhuasts

 

Try rockymountainatv.com for things like nerfs and exhausts. You can also try eBay, that is where you will most likely find an airbox lid for a 300ex without buying from a dealer.
Doughtful that parts for a 250ex or 400ex would fit your 300ex.
You can order a catalogue from Rockmountianatv also.