Guitar makers have advertised in music magazines such as Down Beat, Guitar Player and very many more. Some manufacturers advertised widely (Guild, Gibson, Fender) but many more didn't and adverts are few and far between. Here are some of my favourite ads out of the ones i've seen. You can browse through all the images from the category links above, or by manufacturer from the links below.
AMPS
Ampeg, Leslie,
Marshall, Orange, Peavey, Sunn
GUITARS
Epiphone,
Fender,
G&L,
Gibson,
Gretsch,
Guild,
Hagstrom,
Harmony,
Martin, Music Man, Rickenbacker,
Vox
$1999