It's the "You're NOT paying US to do the repair..well you'll pay thru the nose anyway" syndrome.
I had a LBS hit me with a $30. 'service' charge above the adjustemnts to install the 12-27 cassette on the Moto..even tho I bought the Sram 10spd chain thru them.
(sort of offset the savings gotten thru ordering the cassette on line last winter)
On the flip side, these are the same folks who-although the bike wasn't bought from them-straightened the derailleur hanger and readjusted the 6600 derailleur for $10 (and in 10 minutes)after I lamely laid the bike down on it's derailleur.
I have 3 5 tier bins of small parts I've either acquired thru teardowns and salvages or outright bought from the Gillmores at Loose Screws. Always bought 6 if I needed one. Save the shipping cost and price increase of 'next time'
That said: The new stuff is changing so rapidly one can't acquire used components to hone ones skills on (as we could back in the day)
All that said:
IN Western New York there's a community bicycle work shop (and I'm certain other cities/areas have similar )
and they've been invaluable in finding odd ball parts and 'only need one' items.
Ex: searched high and low and even eBay for a 36 hole 7spd cassette style hub.
Found it at this work shop...$5.
Uncertain if it's nationwide: Try Googling Blue Bike and your citys name)