Most tuba jobs of any importance, both large and small orchestras, both national and international, are listed in the International Musician, the monthly journal of the American Federation of Musicians. If you join the AFM, you get the IM as part of your membership fee. If you don't want to join the AFM, they offer annual subscriptions of the IM for $39.00 in the US and $54.00 in Canada.
My opinion for what it's worth...