Where can someone go to find ideas for fancy dress costumes. There are some great sources on the internet. The one you choose depends on what you like. One of the fanciest is a pirate costume with its ruffled shirt, big hat and pantaloons.
If you are invited to a theme party, that limits your choices. It would be in bad taste to show up in an outfit that did not fit in with the theme the hostess specified on the invitation. Dresses from a period such as the French revolution are quite elaborate. You would not dress like Micky Mouse if the theme was a sixties flower child era.
Your taste may run to the garments worn by kings and queens in merry old England. For a theme party that specifies you showing up looking like a specific movie star, the choices are endless. You could be anyone from Princess Leia of Star Wars fame to Scarlett O Hara from Gone With The Wind. A man could dress as Al Capone in a striped suit with a big black hat and big cigar.
A man can be a vampire for the night in a long black cape with red satin lining. He might wear black suit pants under his cape. He could buy fake teeth, put a little fake blood on his lips and be Count Dracula. The female version wears a long black gown with an uneven hemline and the same teeth and blood on her mouth.
You could buy a French maid uniform with a short black skirt, white blouse and an apron. Carry a feather duster to complete the look. A man could buy a female outfit and shoes in his large size and go as a woman. Add make up, earrings and other jewelry. A woman could put on a masculine business suit and tie with a button down shirt. Complete the look with a glued on mustache.
Some of the fanciest costumes are wild tie dyed shirts and wide bell bottom pants like those worn in the sixties. Wear a short psychedelic go go outfit, along with a pair of white knee high boots. Your beads would be made of glass or plastic in another bright color.
The question, where can someone go to find ideas for great fancy dress costumes, can have many answers. Look in the history books. Kings and queens wore some of the most elaborate gowns and waistcoats and pantaloons. Jeweled crowns topped their privileged heads. You can buy a fantastic garment online. It is possible to look authentic without spending a fortune.