Fingering Charts for All Band Instruments - This is a great site with fingering charts for any wind instrument, and even one for mallets so you can find pitches on the instrument. It shows you both the note on the staff AND where the note is on a piano layout.
Remember that where the notes are identified on the piano are the correct CONCERT PITCH note names; however, if your instrument is a transposing instrument (clarinet, horn, any saxophone, or trumpet), the pitch that SOUNDS is not the same if you play it on a piano. Playing a Bb on a piano won't sound the same as playing a Bb on a clarinet, for example.
Music Theory.net - Lots of great exercises on almost anything on which you need practice!
JW Pepper - The largest sheet music retailer. The website is also useful as it has many band recordings available for listening.
Stantons - An alternative to JW Pepper... once in a while, they have better recordings than Pepper's, and sometimes ones that Pepper doesn't have at all.
Woodwind Brasswind - Looking for band accessories? This is one of your best places to find them. Good prices on reeds, as well!
Amazon.com - Also a great place to find accessories, such as reeds or drumsticks.