I spent way too long looking for base pieces in multiple colors that make a good starting point for homemade baby or toddler costumes. These are all available via Amazon Prime, so you can still get them in time without fitting in a run to the store. Here you go:
A baby pilot cap in BLACK for $10. Boom. In 6 months size, they also have blue, yellow, and pink. The nice thing about these is if you pin costume embellishments to them, you can reuse.
If you need a shirt, it’s cheaper to just order a pack of long-sleeved onesies. I’ll use the rest of the colors anyway. These are $16 for four, and come in a bunch of colors: red, black, brown, blue onesies, and tan, purple, pink, navy onesies.
Leveret baby leggings also come in a zillion solid colors, which supposedly match this brand’s onesies, according to a few reviews, but ymmv. Two for $12.
If reusing the costume as clothes is a main goal, consider Primaries, which are just solid-colored kids clothes in every color. Great quality. It’s most of Ozzy’s wardrobe.
Happy Halloween, guys!
Mighty Girl 
Another tip, footedpajamas.com — they have fleece footy pajamas *with hoods* for toddlers in all sorts of colors. Turning a green one into a dinosaur costume for my 21-month old daughter this week! (With the aid of felt, buttons and a hot glue gun.) She roared when she saw me adhering the spikes. Their products are also available on Amazon.
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