If you need an actual HDD rather than compact flash, you'll probably need to find one on eBay. You might also have luck locally on a site like Facebook Marketplace. It's also getting harder to find new old stock, so you might have to gamble with used.

What brand and model laptop are you working on?

Do you have the original mounting hardware for the HD (some are very proprietary).

Printed on the old laptop ide drives should be cylinders, heads, sectors but most machines just detect the drives since they are in LBA era.

Ebay would be my guess for such a drive, but if you can't find one PM me, I have a bunch.

Depending on how old the laptop is might have to program the harddrive into the bios too

Well...the laptop has a 486DX4/77MHz processor, and I think it has a slot for a hard drive. It shouldn't have LBA. Unknown_K, I thank you for your offer, but right now, I'm not ready for the drive yet: I'm not at my mother's house right now, and the laptop's not hooked up. Sorry! 🙁

77MHz? Doesn't sound right...

That aside, when it comes to laptop drives, there's a whole can of worms to deal with.

Just for the physical drive:
- which interface does it use? (IDE? SCSI?)
- which size drive is it (5.25" wouldn't be a thing with 486, but 3.5", 2.5" and 1.3" are all possibilities)
- which connector does it use (40p IDE? 44p IDE? something nasty and proprietary?)
- does it need a caddy? If so, do you have one?

Then for the logical configuration:
- what BIOS options do you have for configuration?
- do you know the exact original drive that would have been in there?

Most of these questions can be answered using the service manual of the laptop, so it would help a lot if you could give exact brand and model (the full complicated one on the sticker on the bottom, not the marketing name on the top of the case).

I'm sorry: 75MHz. I think it uses IDE/2.5". Again, I'm not at my mother's house right now, and the laptop isn't hooked up, but the next time I'm there, I'll try to remember to look for the information. Thank you. 😀

I think 350mb to 500mb HDD could be too expensive (and small too) for your 486, they more developed for 286/386 era laptops.

I recommend something cheaper, more reliable and bigger than 800Mb, for example:

https://www.ebay.com/itm/315373856464

I have some extra too, pm me if interesting.