Ebay. At any one time you can source hundreds of lifetimes of 7,8 and 9 speed parts, while not being forced into 'upgrading' to the 10 or 11 speed planned obsolescence nonsense.

So if you need a bigger 6 or 7 speed freewheel, they are there in the hundreds. Need larger 8 or 9 speed Campagnolo cassette cogs? Harder to find, but still out there.

Ebay is where I found my supply of 30 and 32 tooth 8-speed Campy cassette cogs. And my 7-speed era Victory compact crankset.

To get lower gears, there is no need to add a few unnecessary cogs to your cassette, or pay hundreds of dollars for an 'upgrade'.