I don't know if this will help much but for FWIW, I gave up on ordering specifically like that pretty early. After a while I realized that I just need to have almost everything I need in stock and any other way doesn't work (for me). It sounds like you repair more instruments than I (never had 250 at once) so maybe even more for you...?What quantities of pads and parts would you guys order for this sized consignment?
Ideas for things like spring assortments, spare pivot screws etc?
I was lucky and had another experienced repairer tell me their pad orders which were in big amounts, so I could see ratio of amount per size. Though this depends on what instruments you see. I started with a decent order of pretty much all sizes with similar ratios. I then gradually learned what sizes I need more and eventually went with another full order, with amounts adjusted to what I thought I needed (not that different from original). Since then I just went with smaller orders to fill my stock when necessary.
That was relatively in the start of repairing for me, when I also had far less work than I do now, so in that year (or two) expenses to profit ratio was a bit ridiculous... that is common for a lot of businesses I think. That investment was worth it at the end and at least for me the best option. I guess I wrongly assumed you were a self owned business, but if you have a boss and work for them, with them paying for the stock, I'm not sure how it would work.
"Unfortunately" later I realized I want to stock another thickness, another type of pad, etc.......
For springs I agree with hornfixer and would also just get some of each size (or most sizes since you might not need very thick ones for example). Compared with pads, replacing springs is rare so it is likely to be a small drop in comparison with the pad orders. Also in comparison with tools costs IME.
For pivot screws, I find I also need very few in comparison with pads. But I came to the same conclusion and just bought a pretty complete stock from a few suppliers. It's not so cheap but luckily it's a relatively small stock, takes very little space and even a small stock can last many years (unless you get a certain model always with missing screws for some reason).