Just watch on Gumtree. These machines are very old now, but in their day they were commonplace, they will still be around. I look at Gumtree only occasionally, usually just for a look, not to buy, and I would see an old Whirlpool about once a month at least. They are usually dirt cheap, sometimes free. Don't just search "Whirlpool" because people are often hopeless and don't list a brand, so look at searches like "old washing machine" and "cheap washing machine" and you can even select "price type" in the left hand column and then select "free" so you can find machines being given away. Some of these will be old belt-drive Whirlpools.

It's useful to know that these machines, although great survivors, were not particularly reliable, and could have any of a multitude of common faults. This is actually a good thing today, because you can collect two or three similar cheap or free machines, and chances are they will each have different faults, leaving you able to make one good machine out of two or three parts donors. 

Some other machines have one or two common faults, you could collect 10 of them and they would all have the same fault, which makes them useless for donating parts.