Over the course of many years, I have found that it is difficult to find useful information...
Below are some websites I would recommend for personal use:
http://forums.chisham.com/ This is a forum based website that has used/new instruments for sale, auditions both professional and school based, and other useful information for tuba/lowbrass
https://www.musicalchairs.info/ A job database regularly updated for an available job for any instrument, tuba included!
http://www.iteaonline.org/index.php The "tuba/euphonium" website that every student MUST be a member of. With its membership, it includes a variety of videos and resources that are applicable to everyday activites. It also includes recent job hires, great school information, recent events, and various info on national and international competitions.
http://www.potenzamusic.com/ A great website to buy solos, and ensemble music for a variety of instruments.
http://imslp.org/ A database for a large majority of public domain sheet music available for immediate use/print. This is a great website for all musicians, whether vocal or instrumental.