Where can the following quote be found: "Elizabeth saw with admiration for his taste, that it [the furniture] was neither gaudy nor uselessly fine; with less of splendour and more real elegance"?

The quote you mentioned can be found in chapter 43 of the novel. In this chapter, Elizabeth visits Mr. Darcy's Pemberley estate with Mr. and Mrs. Gardiner. For her part, Elizabeth feels a little nervous about visiting Darcy's home.
However, she comforts herself with the thought that she isn't likely to see him there. After all, Darcy has other properties, and he is a well-traveled gentleman of means. Despite her feelings about Darcy, Elizabeth can't help but admire his taste in furniture. She notes with admiration that the furnishings are neither gaudy nor "uselessly fine." Instead, they are elegant and tasteful.
It is interesting that the chapter begins by highlighting Darcy's taste in home furnishings. This focus foreshadows a change in Elizabeth's opinion of Darcy as the chapter ends. Mrs. Reynold's warm praise of Darcy also plays a major role in changing Elizabeth's mind.
The coup de grace occurs when Darcy himself shows up at Pemberley. In behavior, he is the personification of dignity and grace, and he manages to charm everyone. Additionally, his awkward but charming request to introduce his sister to Elizabeth immediately disarms the latter. By the end of the chapter, Elizabeth is not quite sure whether Wickham has been completely honest with her (about Darcy's bad character).