You can find all kinds of commercial embellishments to finish that special card, but if you want to be truly unique, bypass the ready made embellishments, and dare to be different. Some of my favorite places to search for one of a kind items for my cards are flea markets and garage sales. They're also the best place to find alterable items.
To create successful art you must see things not as they are, but as they could be. Look for vintage photographs and postcards. You'll be amazed at what people are giving away for almost nothing. Vintage ephemera - newspapers, magazines, books, menus, maps, travel brochures, etc. can all be used in your creations. Don't overlook things like small hardware and jewelry pieces. They can be attached to almost anything. Rip things apart - don't always look at the whole piece - there are some extremely interesting components to other people's "junque". Old games - scrabble, dominoes, mahjong, etc. provide alterable items as well as embellishments. Look for inexpensive books, records (if you're old enough to remember what they are!), boxes, tins, clipboards, picture frames, and anything else that you can recycle.
Some other unusual embellishments that I enjoy working with are cancelled postage stamps (make sure to watch your mail!), flattened bottlecaps, buttons, mesh bags from vegetables, broken jewelry pieces, puzzle pieces, playing cards and fishing items (not the hooks!). Keep your eyes open for any small item and save it - it might come in handy later. Using these types of embellishments will make your cards and other creations truly one of a kind.
One final thought - my rule which I have broken too many times and later regretted it:
If you pick something up twice it better go home with you.
Leave it behind and you'll be sorry later that cheap treasure got away!