Right here http://www.nostalgicairparts.com/ac/compressors-41.php
second one down, the 709 clone $185 brand new. You have a 508 but the 709 is a direct fit for a 508 and it's a much more efficient and larger capacity ( 508= 5 cyl 8 cu inches, 709= 7 cyl 9 cu inches )compressor. I've bought a bunch of compressors from that guy and never had a single problem with any of them.
Make sure to check which oil your system has in it ( if it came with r134 from the factory it should have pag, if it was a retrofit with the volvo kit it should have ester , if it was a gumby brand retrofit, it's anyones guess and if it's not marked you better flush it and start from scratch )and tell him and he'll pre fill the compresor so you will not need to add any oil to the system. If the old one seized up, make sure to flush the system thoroughly before installing the new one.
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-------Robert, '93 940t, '90 240 wagon, '84 240 diesel (she's sick) , '80 245 diesel, '86 740 GLE turbo diesel, '92 Ford F350 diesel dually