After some eBay, Etsy, Pinterest, and Google Images exploration, here's what I came up with.
these on eBay, but he'll wear just the top for Halloween.
White sweatpants - They'll be warmer and look more like a costume than the PJ pants.
Winter boots - In a few weeks I'll do the yearly winter gear stock-up, aiming for white or grey boots for Michael with the costume in mind. Buzz's toes are green, but I think this will work fine.
Inflatable wings - Michael tried the wings on this morning and was thrilled! Luckily he was cool about taking them off to be stored safely for over a month until our first Halloween event.
The Buzz costume parts are all re-wearable aside from the wings, and the whole thing requires no DIY, assembly only.
Red hat - A cowgirl's gotta have a hat.
White shirt with a yellow bandana - It's not exactly Jessie's look, but it will be cute and more functional later. The shirt was only $6 from Old Navy and the bandana was $2.
Red boots - I didn't intend to go for boots, especially since they'll hardly be seen under the jeans and chaps, but red cowgirl boots were staring right at me when I went on the Old Navy site to get that white shirt. They were on sale and look like this:
I'll do my best get $15 worth of use out of them, perhaps swapping out the jeans for a denim skirt and white tights for one of our Halloween events. Knowing how much my girl loved her brown suede boots last year, I suspect they'll get additional wear. With all of that in mind, I just couldn't resist the splurge.
Jeans and cow print chaps - This is the only DIY-ish part of the pair of costumes. Here's what I've done:
I bought 1/2 yard of "dalmatian" fabric from JoAnn Etc. (already 30% off) with a 40% off coupon. It came to just a few dollars, and there's a lot left over, which is good in case Sophie outgrows those jeans this month.
I just traced around the legs of the jeans Soph will wear, from the crotch to the bottom, adding a little "tab" at the outer ankles for fringe.
I cut the non-fringed sides and bottom with pinking shears (seemed cowgirly to me) and the top and fringe with regular ones.
I traced the edge of a big plate and cut to get the rounded top edges. The straight lines looked too severe when Sophie had them on.
I may or may not sew some ribbon onto the backs of these to tie them onto Sophie's legs with. Pinning with safety pins in between the two layers worked fine but really isn't a ton easier than whipping out the sewing machine for a few minutes. We'll see.
Those are all the ingredients and major steps. You'll have to check in next month to see the costumes in action!