The most comprehensive buying guides for classic cars are usually found in magazines, not on the web. Hopefully this list will help you track down the issue you need. Ebay is usually the easiest place to buy second hand magazines.

MakeModelMagazineIssue
Alfa Romeo156 (1998-2006)Practical Classics2013, September
AustinAllegro (1973-1982)Practical Classics2015, February
Austin / MGMaestro (1983-1994)Practical Classics2017, March
CitroenXM (1989-2000)Practical Classics2016, July
FordGranada MkIII & Scorpio (1985-1998)Practical Classics2017, Spring
Ford100E & 107E (1953-1962)Practical Classics2006, January
FordEscort RS Cosworth (1992-1996)Practical Classics2016, November
FordCortina MkII (1966-1970)Practical Classics2016, August
Jaguar XK120, XK140, XK150 (1948-1961)Practical Classics2016, September
JaguarXJ6 X300/X308 (1994-2003)Practical Classics2014, April
JaguarS-Type (1998-2008)Practical Classics2017, June
JensenInterceptor (1966-1976)Practical Classics2014, Spring
Land RoverDefender (Heritage, Tomb Raider & G4)Land Rover Owner2016, August
Land RoverRange Rover Sport (2004 - 2013)Land Rover Owner2018, Spring
Land RoverDefender 300Tdi (1994-1998)Classic Land Rover2018, May
LotusEsprit (1972-2004)Classic Car Mart2012, January
LotusElise (1996-2000)Practical Classics2014, August
MazdaRX-8 (2003-2010)Practical Classics2017, July
MazdaMX-5 MkII (1998-2005)Practical Classics2016, October
Mercedes190SL (1955-1963)Practical Classics1985, December
MercedesSL R129 (1989-2002) Practical Classics2016, June
MGMGC & MGB GT V8 (1967-76)Practical Classics2017, April
MGRV8 (1993-1995)Classic Car Mart2014, May
MorrisMarina & Ital (1971-1984)Practical Classics2016, Spring
NSURo80 (1968-1977)Classic & Sports Car2017, July
PorscheBoxster (1996-2004)Practical Classics2014, September
Saab900 convertible (Classic Car Mart2015, October
SmartCity (1998-2007)Practical Classics2017, August
SunbeamAlpine (1959-1968)Practical Classics2015, June
TriumphDolomite (1970-1980)Practical Classics2015, July
VolkswagenType 2 (1950-1979)Practical Classics2005, July
VolkswagenGolf GTi MkIII (1991-1999)Practical Classics2016, December
Volvo140-260 (1966-1993)Practical Classics2017, May
Volvo850 / T5 (1991-1996)Practical Classics2017, January