I like buying my soft goods like mattresses and couches new and things like dressers, tables, and headboards secondhand because the quality is often better than what you can buy new (real wood over veneers).
The most interesting spaces will bring in some vintage or “character” pieces, so I’d hit the thrift stores or family attics to see what you could repurpose. This will go a long way to keeping your prices down, too.
I like Article for good quality, modern design furniture. My boyfriend and I fight over who gets to sprawl on the Sven sofa we got from them. 
Not a source, but an immense help in designing my place, was @1chicretreat. She does a wonderful job of mixing high and low-end things in a way that comes off very polished and beautiful. She has excellent advice on everything from lighting to what makes a good bedroom.
Her “wildflower technique” is simply the best article I’ve read on what makes good design.
Here’s her article on the cost of furnishing a master bedroom which looks like it might qualify as the “modern, premium style” you’re looking for.