I'm also finding this the one of most expensive/difficult parts of start-up. I keep 5 - 10 yards of black, plain domestic coutil for use in sample/experimental work. I order white/imported on an as-needed basis. (Which I plan to remedy with my next order, so it will be on-hand.) I keep a variety of busks, based on the next 10 corsets I plan on making. (Usually, no more than 10 in the works, due to cost.) I focus on two styles at a time, for example Edwardian long-line and waist-cinchers, so there are 5 of each busk length on hand, minimally.
I have on-hand a large supply of fashion fabric to draw from, but have found that my customers often have something specific in mind when they order, so keeping fashion fabrics and novelty prints in stock has been a poor option, over all. Best to stick with solid silks and perhaps some spot coutil selections.
I keep my silk wrapped in a dark closet to avoid light (natural or artificial) and have a few colors, but only because I obtained a great price on a couple of rolls. Usually, this is ordered on an as-needed basis, but can get harrowing when there's an order to be filled by a certain date.
I keep gold and silver grommets in 2 sizes on hand, and order by the gross. For boning, I cut my own steels and order by the roll - 10 yards at a time, always saving up for the next order as I go. I do keep common lengths (such as 7.5") on-hand and order by the gross to save my hands, somewhat, as cash-flow allows.
I have not quit my day-jobs, but work in environments that support my corset-making. The income continues to pay for supplies.

You could specialize or brand-ize.

Focusing on waist cinchers would limit the size range of busks you need on hand.
Single layer corsets require less fabric.

Who is your target audience?
Goths: black coutil and silvertone findings
Steampunks: brown (leather) and brass findings

Fashion Fabrics?
I generally do OOAK custom work - hands on. I pick up random yards of fashion fabrics and add them to my "kit" of options for customers to feel and choose from. I generally have between 20 and 35 at any given time.
If you are offering fabric choices online keep it as simple as you can afford. If you offer red satin coutil for example, be prepared to see a rise in orders for red items near x-mas and valentine's.

How many pieces do you expect to make in 2 months?
Consider your most popular items. Try to keep this amount of stock around at a minimum. You never know when you'll have a supply chain issue (or get a package of supplies stolen or lost in the mail).
I have plans on expanding, but for now, I'm working on a really small scale (2-4 a month). I keep the supplies I need for the few styles I push. When I only have enough supplies to make 6 of anything - it's time to reorder. I gladly accommodate customers' interest in other styles, with the caveat that it will take time to order parts I don't have. I can always make a mockup with a solid front or a strong zipper while I'm waiting for an emerald studded busk to arrive.

You could always go the ready -to-wear route, separating the customer from all supply chain issues entirely.

Hope this helps!

This is what I usually do if two things are starting to run low I'll try and use one sparingly so I don't end up having to spend out on a lot at one time.

Thanks!

I will stock up as $$ allows!

Kayla

I have a 5m roll each of flesh coutil, black coutil, black floral patterned coutil and red satin backed coutil.
I have 20 14" long silver busks for overbusts which I cut down if needed to be shorter and 20 10" long busks for underbusts and I have 10 14" long black busks.
I also have hundreds of rivets and enough swinghooks in silver, black, gold and antique gold for two corsets each.
I also have 25m rolls of 7.5mm and 5.5mm flat steel and 4,5 and 6mm spial steel, and 1000s of eyelets.
I learned from giving people a quote of 8 weeks and something getting stuck in customs once so I couldn't make the deadline.

Just a quick daft question....

I am just wondering from the professionals corset makers... How many busks of each size do you keep n hand or do you order as required? Same for coutil, do you keep x amount on hand or order as required.

I ask because I am wanting to slowly throw in the day job to make bespoke corsets. Whilst we have suppliers of hardware here in Australia, they can end up being expensive and have found it cheaper to import but that can take anywhere from 1 week to 4 weeks to arrive.

Thanks
Kayla