In search of 105 8 speed cassette.

Shimano 105 was once 8sp, then it became 9sp, 10sp and now it's 11sp.

So the question is why do you specifically want a 105 cassette when the HG50 is probably exactly what you have and any 8sp cassette would fit.

Fit one of these and be done with it:
https://www.wiggle.co.uk/cassettes/8-speed-speed 

Add a wanted post on retrobike.co.uk in the road section, make sure you know what you would pay.   There is also an losely affiliated site on facebook of the same name where most of the regulars also hangout.

Ebay for NOS cassettes, it's where I tend to source my 8 speed XT and 9 speed XTR.  You have to keep your eye on it and expect to pay premium for what are becoming very rare consumables.

I happened upon this site today

https://www.retro-himmel.de/

They seem to have been hoarding box fresh goods from some time ago and are charging rather a lot, or stuff he picked up a little while back when relatively cheap.  Lots of very colourful 90s mountain bike candy.

Sometimes it seems like a quest, lots of google searches, and going through many pages with different search phrases.  Get an exploded view and there may be a few single cogs out there if you don't need the whole thing.

There does seem to be more parts available in the US, and mainland Europe.

I have never chanced second hand cassettes or rims.

On chainsets and chainrings wear is much more obvious.