I never want to ruin a good costume by having to carry around a regular purse, and stowing my id and money in my bra doesn't work nearly as well as it did in high school. Here's my solution; the prop purse!
First, find a stuffed animal that fits your costume and would easily hold your wallet and phone, (or whatever else you need to have on you,) in its belly. Thrift stores are often overstocked on stuffed animals, so you can get them there for dirt cheap if you're not opposed to their questionable origins.
Hopefully, your animal has a seam that runs along its back (or wherever you want your zipper to be.) If it does not, you could probably just cut one in and ease the fabric to put the zipper in, but for the purposes of this tute, we'll assume a seam is there. Now get your seam ripper and open it up. The body of my flamingo is pretty small, so I went from the base of the neck almost to its tail. Remove a handful of stuffing here.
Next, you want to measure how long your opening is to make the lining. Now you'll need to find a zipper the right length, or trim an existing zipper to fit that length.
It's also a good time to make the inside lining. Since my seam length was 6", I cut out a piece of fabric that was 7x4, to account for seam allowances. The length of the lining was determined by eyeballing how deep I thought the body might be.
Sew the sideseams of your lining, and trim the edges.
Now, take your zipper and start sewing it in, right side of the zipper to the right (fuzzy) side of your animal. I usually start with the zipper unzipped to maneuver a bit more easily.
When you get towards the end of the zipper, you may have to zip it back up to sew under the zipper pull.
When you reach the end of your first side, knot your thread. You might have a bit of stuffed animal that the zipper didn't cover, we'll sew this back up later. Starting back at the neck area again, start sewing down the other side of your zipper.
This time, when you reach the end of the zipper, continue sewing to cover any areas that may still be unstitched, and knot the end when you've reached the end. You'll notice I became frustrated here, and after ripping out the insides of this poor animal, I turned much of the body inside-out to get the the tail area stitched.
Now, with your zipper sewn in, place your lining inside the hole of your animal with the edges pressed to the outside. Mine are not pressed, obviously. My iron and I often have misunderstandings.
Begin sewing your lining to the seam allowance of the zipper. I sewed mine in a full circle, so that it was just one seam.
All that is left to do now is attach some form of handle or strap if you want one. For the flamingo I chose to just use a small chain, which I sewed on at the neck and tail.
Now stow things away, and prepare to get compliments!
This purse was made for an Alice in Wonderland costume that was worn last year, but will be resurrected someday, just for this purse.
Here's the final product with a human. I have no idea why I looked to angsty. It was probably the awesome sunburn.