Headset washer?

[The Path Racer]

In my infinite wisdom I've managed to squish n' a squash the thin plastic washer that sits immediately below the lower caged bearing on my 90's mountain bike. I'm not sure of it's purpose but I guess it prevents the grease from migrating. The question is whether these types of individual items are available or do I need to buy the whole set of component parts?

[Brucey]

well the good news is that you probably don't even need this bit [ I think it is a seal from what you say] as long as you also have something [anything] to prevent crud from the front wheel from getting into the bearings. The bad new is that unless you get lucky or are prepared to whittle your own, finding a replacement part is likely to be a problem.

[rjb]

An old inner tube can be cut up to yield a suitable cover. A cable tie to keep it in place may help too.

[Cyclothesist]

Lizard Skins make a neoprene headset protector cover that velcros closed at the back. Its designed for skinny head tubes such as found on a 90s MTB. If you're handy with needle and thread you can make your own.

[The Path Racer]

Oh well. I do have a circle cutter, a hand fretsaw and a bunch of half round files, oh and some guitar scratch plate material. I doubt it's the same headset type but there's also an old Apollo that I was thinking of using and stealing the fork for a truing wheel stand.
Would a rubber 'O' ring work?

[Brucey]

yes an 'o' ring might work. I would always try and recover the old part; maybe if you immersed it in hot water it [ if really a 'shield' made of hard plastic] might return to its original shape? I have made replacements myself, [or added shields where there were none previously] mostly by cutting up food packaging with scissors.. My 'ersatz' parts were usually a 'c' shape [ ie having a gap ] rather than an 'o', but they could also be an ' L'section too, so they worked quite well as shields.