My wife and I are foster parents coming up on 2 years now. As we've come to realize, our state essentially thinks foster parents are doormats and have pushed us to the edge where we are going to have to really think about continuing this after our current placement goes back to her mentally unstable mom (at this point it's not an if, but when, unfortunately).
Be prepared for the hassles and absolute frustrations that come with dealing with what is likely an underfunded program with workers who are likely underpaid and overworked. Not sure how Missouri works, but in our state there are basically 2 branches of the organization that we deal with - one side is our placement worker who is essentially our advocate. The other side is the worker for the biological parents/their kids who are in foster care. We have been lied to numerous times by them, my wife has been yelled at over the phone at least twice by them, and we are constantly advocating for the best for this kid. As I said before, this kid's mom is nuts and in our state, it's clear the biological parents have way too much say when their kid/kids are in foster care. Example, our current girl just turned 3, we've had her since she was 18 months old, and we still haven't been allowed to get her a haircut because her mom has to approve it, and she won't.
Document EVERYTHING through e-mail with all the relevant people in the organization. Even if it's from a phone call or in-person visit, follow those up with e-mails just so there is something to refer back to at a later date.
I wouldn't worry at this time about stocking up on anything. Like other's have said, you're going to be at the store the day of/next day you get the kid. Hopefully they give you some notice and you can maybe get to the store before they are brought to you. Or you could be suckers like us who thought we were just being nice when we said we'd take 3 kids (5, 4, and 10 months) "just for a 48 hour emergency hold" at 6pm the day before Thanksgiving right after we finished training. No formula or diapers for the 10 month old and the other 2 only had the clothes on their back. Ended up keeping them 6 days (because 48 hours isn't really 48 hours in our state's world). Oh, and the case worker who dropped them off went radio silent the moment he pulled away from our driveway.
Lots of great positives about fostering, but also realize it's not all sunshine and roses like one would think. Depending on your state's laws, you may not be able to parent exactly like you think you want to.
I would suggest looking into reading some parenting books, especially if they are geared towards really hard kids (likely not the kid's fault, just their situation).
Also be prepared for your schedule to fill up, and fill up very fast. Visits with their parents, sibling visit, extracurriculars, therapy, etc.
Make sure to ask your case worked about what is reimbursable and how to submit those expenses. We've been pleasantly surprised what types of things get reimbursed for our kid.
I don't mean to come off as super negative, but our reality has been the only positives all come from the kid themselves. Expect a net negative from your state and be happy if it's better than crap.
You're more than welcome to reach out to us if you'd like. LavenderGooms012 at gmail
They have been an absolute blessing for us