How big? We actually have an LFS that pretty much sells tanks at cost. They figure they have you on the hook for all the "extras", where they make their money.
I bought my reef tank from saltwateraquarium.com and was great to deal with. Bulkreefsupply and marinedepot.com also great but haven’t bought a reef tank from them.
LFS is the way to go if you have one that will order the tank for you at cost. I saved a few hundred and got my tank delivered, (into my house not on the curb) with the stand set up (for a small addition fee of course). Even with the setup fee, I still paid lower than the online price of my Red Sea Reefer 750 XXL.
Also, the tank came in a few days not weeks, (delivered to the LFS not me) . I did have to wait a couple more days for them to schedule the delivery and setup the stand, but I had the tank in my house on the stand in 9 days from the date it was ordered!