I want to play a game as a mad scientist character. I need some stylish lab wear (befitting a lady mad scientist) and assorted lab decorations. Other spooky/steampunk items and outfits are welcome too!

I'm gonna just assume you know about the Steampunk collection at the Store, which is...like...the BEST.
http://store.thesims3.com/setsProdu...ategoryId=12084
Midnight Hollow is the defacto Spooky/Steampunk world, and it comes with lots more stuff, including more CAS stuff, objects, and really cool rabbitholes.

TSR had a long-running Fratres thing going on for their Steampunk theme -- lots of stuff for CAS and lots.
http://www.thesimsresource.com/down...earch/steampunk
http://www.thesimsresource.com/them...skipsetitems/1/

Right here at MTS is some splendid stuff, including jaw-dropping lots:
http://modthesims.info/browse.php?t...&gs=2&f=38&gs=2
But you asked specifically for mad scientist stuff, so you'll NEED this amazing set: http://modthesims.info/d/423662

Then, browse the Industrial/Urban Decay list for all sorts of goodies, ESPECIALLY Skeletal Screams' Broken World stuff:
http://simswiki.info/wiki.php?title...l.2FUrban_Decay
http://simswiki.info/wiki.php?title..._Themes#Trashed

Browse the Steampunk tag for more stuff: http://scifits3ccblog.tumblr.com/ta...ims+3+steampunk
Because you want mad scientists, OP, you might also get a kick out of the Medical tag running there, as well, for more lab equipment and medical apparatuses. http://scifits3ccblog.tumblr.com/tagged/sims-3-medical

And if you're interested my steampunk conversions are all here: http://murfeelee.tumblr.com/tagged/sims-3-steampunk

You'll go a long way if you have the Ambitions and Showtime EPs, or even Into the Future, since they all added extra gidgets and gadgets, especially Ambitions.
Parsimonious made a plain Workstation (thank heavens) here: http://www.parsimonious.org/furniture3/pages/all_1.html

That's a lot of stuff to go through! I'll start sorting through it right after I cook dinner.

EDIT: That's certainly a lot of "steamy" stuff, but not so much "mad scientist". While I do love the steampunk aesthetic, I prefer things to be functional. The outfits are nice, but none seem to fit lab safety codes. The venues/houses are nice, but they don't seem foreboding enough.

I want items that wouldn't look completely out of place in Moonlight Falls.

You'll need to post some pictures of things you have in mind.

Foreboding as in...horror?
At the forums there's a whole Master Post for lots people shared for Midnight Hollow. As I said, that's the go-to for spooky-steampunk, so a lot of lots and labs and such are over there; many 3rd party CC free.
http://forums.thesims.com/en_US/dis...aring-thread/p1

Lab safety codes?
There are lab coats aplenty in the Medical tag I linked you to, and the base game and Ambitions have the career outfits for mad scientists, regular scientists. and doctors.
The Movie Stuff Pack also came with a slight handful mad science CAS stuff, but it's way more comic book villain or Austin Powers style; I don't know what aesthetic you're gunning for without picture examples.

Anything beyond the EP items are often relegated to coffee machines, stoves, computers...small things like that. And you can find all of those in the TSR Steampunk link I gave you.
Here's another one: http://digitalperversion.net/garden...6&topic=23163.0

Functional items are few and far in between, unless you have Ambitions, Showtime, or Into the Future.
For instance: this Jules Verne set is loaded up with steampunk functional items, but if you don't have the EPS, it's not gonna work: http://aroundthesims3.com/objects/r...wntown_22.shtml
On the other hand, base game friendly sets like these are just decor items: http://aroundthesims3.com/objects/r...wntown_16.shtml

EA hates variety, clearly, so you have to get creative with disguising/dressing up the few functional items we have to look like something unrecognizable.

There's lots of medieval torture equipment out there, but again, it's all practically 99% decor.
http://digitalperversion.net/garden...6&topic=22337.0
http://www.thesimsresource.com/down...ber/id/1038295/
http://digitalperversion.net/garden...6&topic=30523.0
http://www.thesimsresource.com/down...ine/id/1011837/
http://digitalperversion.net/garden...34510#msg534510