Midwestcustomaquariums.com
I have never read a negative word about these tanks. Chris, the owner is a great guy and great to work with. He definitely went the extra mile for us and actually found an appropriately sized wet dry filter on eBay for our 300 gallon tank.
Fishtanks direct... I purchased our big tank through them. The salesman was pretty bad...he no longer works for them. He basically lied to me by omission. Doviiman purchased his 800 gallon rank from them and had a positive experience. Things are different now though as they are located in Florida instead of Arizona, and I think they build their own tanks instead of brokering tanks through different tank manufacturers.
ATM acrylic tank manufacturers is another builder. For a 300 gallon tank, I'd go with Midwest.