Ditto looking online (craigslist, freecycle, ebay, etc)
We only usually get 1/2 a costume ;)
Which only makes sense, if you think about how cold and nasty it usually is on halloween here. It's not like we're in SoCal with 70 degree clear weather on halloween that can deal with thin costumes, or costumes where there's skin showing. We need polar tech, umbrellas, and sweats!! So half of his costume is ALWAYS either a pair of sweatpants or a hoodie that he'll use for the rest of the year. When he was Harry Potter, we actually got double the use...because we went with the polartech hooded quidditch top (which is just a sporty maroon with yellow striped hoodie), in addition to the sweatpants we bought him. Lighting bold scar with eye liner, broom made from a branch out in the yard, and crafting straw from Michaels rather than spending 50 on the costume and 30 for the broom. We paid about 10 bucks (in stuff that wouldn't be used again and again).
You can "dress up" the part that isn't costumey, so it fits with the costume. Obviously a matching color, but then you can get creative...
When he was a lion, I used a big fat drapery tassle and braided rope as the tail tied around his waist, and the top part of the lion costume (came from the midwest, it was a actually filled with down, how warm and snuggly is that?), rather than buy the bottoms that came with the costume that had the tail attached... Drapery tassel from pacific fabrics: 2 bucks, and braided rope 1 dollar per yard, used less but spent the buck. 2nd half of lion costume: $25.
For my nephew, we used elmers glue (washes out in the machine, no worries at all, doesn't even stain :), and glued scales made of holographic plasticky wrapping stuff on his sweat pants to help with his dragon top, and for my neice we made papermache seashells (painted with acryllic, so they wouldn't melt) for a mermaid costume one year...another year it was fairy wings (nylons, spraypaint, glitter, feathers, two garters for arm bands, and wire hangers...although you can just use nylons and wire hangers and paint),and a french twist and makeup over her normal clothes.
Good Luck