SOURCES:
mostly ebay, plus mainly these vendors:
- 3000 Toys
- Troy's Toys
- Global Diecast Direct
- TRAINZ
- and a few others

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REFERENCES & LINKS::
These are just the ones I use most, led by Google searches.  
There are many more.

http://catalogs.simba-dickie.com/en/home/
SIMBA DICKIE GROUP
The Simba Dickie Group is a German toy manufacturer founded in 1982 as Simba Toys.
Jada, Majorette, Schuco, etc.
a fun selection of flippable catalogs of recent diecast issues from their brands

http://hotwheels.fandom.com
Hot Wheels Wiki
99-44/100ths percent of everything you could want to know about Hot Wheels!

http://majorette-model-cars.fandom.com
Majorette Model Cars Wiki
A work in progress - spotty but occasionally useful

http://matchbox.fandom.com
Matchbox Cars Wiki
99-44/100ths percent of everything you could want to know about Matchbox!

http://www.diecastgarage.org
"A forum for collectors of diecast vehicles, with a particular focus on 1/64. "
Forum with endless member-contributed data, imagery and commentary about all things diecast.

http://www.hobbydb.com
Database of diecast toys
"Built by collectors for collectors, hobbyDB is the world's fastest growing collectibles database"

http://www.hobbytalk.com
another forum:
A forum community dedicated to all Hobbyists. Come join the discussion about die cast, models, slot cars, radio control vehicles, classifieds, swaps, and more

http://www.m2museum.com
Micro Machines Museum

http://www.publicsafetydiecast.com
Excellent resource:
Collectors' Guide for 1/64 scale Matchbox, Hot Wheels, Corgi Juniors & Majorette Since 2007
Police, Fire & Ambulance Emergency Vehicles

http://www.swiftysgarage.org
Forum for collectors of diecast cars, with a particular focus on 1/64