Salons place the gown orders with the manufacture based on the largest part of your body (for instance, my hips are much larger than my chest) and some salons probably even add on a size or two to make the alterations substantially costly. If a gown comes in too small and the seamstress cannot let it out enough, the salon has to eat the cost of it and they'd rather not do that.
So your gown comes in and it's two sizes too big and 8" too long. This is not a cheap and easy fix, no matter where you take it. Wedding gowns, because of the delicate fabric, lace, beading, and elaborate details are EXTREMELY difficult to alter. You can't just hem a wedding gown like your dry cleaner hems your slacks for $15.00. Most of the time the skirt has to come off at the waist so that the detailing at the bottom is not lost. Only the most experienced tailors & seamstresses (think 20+ yrs of experience) will even touch a wedding gown. Many seamstresses who work for themselves won't take on the liability of a wedding gown because if she ruins it, she probably doesn't have the thousands of dollars to replace it. If the salon's seamstress ruins your gown, the salon has to pay for a replacement.
I always recommend sticking with your salon for the reasons I just mentioned, but if you do decide to take your gown to a tailor/seamstress that is not affiliated with your bridal salon, make sure you get some solid recommendations and ask to see photos of their work. Be very careful and make sure that you aren't just shopping for the cheapest price.
My advice: set aside some money for alterations so you're not frantically scrambling right before your wedding.