All of the 8" floppy drive manufacturers are dead. With the exception of TTL, transistors, and some microswitches and optical sensors, there is no source for new replacement parts.
There are still companies around (aftermarket surplus dealers) who claim to have factory new drives available, many of those drives are/were rejects from OEM Users that have supposedly been tested and/or repaired. Many of those drives are still defective. If you decide to go this route, make sure that you get a warranty from the Seller that the drive has been tested to factory specs, and that the Seller will repair or replace the drive if it malfunctions for a period after delivery (and I don't mean only 10 days, more like 90-180 days at minimum).
Most of the later 8" drive designs have some custom chips (the late SA-800/800 have a 40-pin custom LSI chip & customer read and write chips, while the SA-850/851 uses the same custom read and write chips as the SA-800/801, plus a custom stepper motor driver chip). As with other parts, the only source of replacements would be donor drives. The Shugart SA-801 and SA-851 are hard sector capable 8" drives. The only difference between the hard sector (801/851) and soft sector (800/851) are a few parts on the logic boards (which are physically the same circuit boards for the SA-800 and -SA-801 OR the SA-850 and SA-851). Most late drives carry the hard sector 801 or 851 logic boards with the hard sector circuitry Option Jumpered Out.
As far as 8" full height floppy drives go, the Shugart drives are the best for reliability and longevity. They are also the easiest to find replacement parts for. The Siemens drives work,but they are slow. Most Qume full height drives are junk (DT8/DT-842) with 90% carrying bad head assemblies, bad logic boards, and/or bad stepper motors. The Mitsubishi full height 8" drives are poorly designed, and not worth owning.
I used to like the Shugart 1/2 height drives the best of the 1/2 height 8"floppies. Like the NEC FD-1165 they are mostly metal and hold alignment fairly well. But this past year, after trying to piece together a pair of Shugart SA-860 double-sided drives, I've found that the design has aged very poorly. Most of the Shugart 1/2 height drives I have are having problems with the stepper motor, and remember there aren't any new replacements. My previous 2nd choice Mitsubishi M2896-63 1/2 height double-sided is now my 1st choice for 8" 1/2 height floppy drives. I've been using a pair for over 20 years without any problems. I've had poor luck with Qume DT-242 1/2 height drives although some people have had good luck fixing the common failed logic boards on these drives. The NEC FD-1165 drives are made with excellent quality for the most part and have an interesting rotary head load mechanism, BUT many suffer from upper head assemblies that get loose (causing upper head to go out of alignment relative to the lower head, this will total the drive (according to the factory) unless a drive repair person with "head splitting" skills is available to try to fix the problem).
I would not pay more than $25-$50 for a Qume drive to use for parts. I would not pay more than $75 for a used Mitsubishi M-2896-63 or a NEC FD-1165 unless it came with a warranty that it was aligned and working (in which case I might go to $150 for an NEC or $300 for a M-2896-63). A working Shugart SA-850/851 with alignment and warranty is worth $300-$400, AS-IS, it would be worth $25-$75. Working Siemens drives are worth $100-$200.
I charge $100 alignment - $150 repair and alignment for a single-sided drive, and $150 alignment - $200 repair and alignment for double-sided drives. That would be plus parts, and plus return shipping. You can get an idea of why I value unknown used drives the way I do. Most used drives that I come across need some attention. Unless you are paying for a drive that has been serviced recently, you should expect to be paying for drive servicing before you can use any drive you buy.
If you find something you're interested in on eBay, I'll be happy to look at it/them for you and give you my honest opinion (free of charge) if you send me the link. Just don't link every 8" floppy on eBay. There are a lot of 8" floppy drives on eBay to choose from, and many are junk, not worth buying, even for parts.
3.5" and 5.35" drives are not made to the same level of quality as many/most 8" drives were. While 3.5" drives are newer, and more available, a larger percentage of them are out of spec, and they are not made to be repaired.
