I need to replace the front shock absorbers on my 1991 560 SEL. Where will I find good performance parts without paying inflated high retail prices. Also, is it advisable to replace only one side?

I am not sure about performance, but I do know you never replace one at a time, always replace in pairs or if the other wears out it will prematurely wear the new one and now you have to buy and replace three instead of two...

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1985 300D Turbo 332k Miles (daily driver)
1987 300SDL Turbo 320k Miles (needs new head)
1968 International Scout 800 V8 119k Miles (garage queen two years and counting)
2005 Kia Optima (backup car) 36k Miles (accident )

try thebenzbin.com,they have great prices and different brands to choose from

I put KYB Gas-a-just on my car all around. I like them, but I hear Bilstein HD are very good also from this forum. I put some Gabriels on my van and they sucked. This is a good price right here. http://catalog.worldpac.com/mercedesshop/sophio/quote.jsp?clientid=catalog.mercedesshop&cookieid=2AG10S9D42CP13X3AR&baseurl=http://catalog.peachparts.com/&partner=mercedesshop&year=1991&product=L4000-10210&application=000230416

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1984 300SD Orient Red/ Palomino
1989 560SEC
2016 Mazda 6 6 speed manual
1995 Ford F-150 reg cab 4.9 5speed manual

Have you tried Fastlane,,,, above ? One of the sites' main sponsors ?

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95 SL500 Smoke Silver, Parchment 64K
07 E350 4matic Station Wagon White 34K
02 E320 4Matic Silver/grey 80K
05 F150 Silver 44K

my thougts exactly. Hence the link above

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1984 300SD Orient Red/ Palomino
1989 560SEC
2016 Mazda 6 6 speed manual
1995 Ford F-150 reg cab 4.9 5speed manual