Are you talking about the rubber boot/seal that's shaped like a Tophat?
http://www.makcotransmissionparts.com/44371B.html
http://www.makcotransmissionparts.com/44373B.html
Pic on this link>> http://www.transmissionpartsusa.com/350_Detent_Cable_Grommet_Seal_p/470-000047265.htm
Some use an O-ring on the Dipstick tube & Detent cable.
http://www.makcotransmissionparts.com/44373A.html
http://www.makcotransmissionparts.com/44371A.html
http://www.transmissionpartsusa.com/TH350_Transmission_Detent_Cable_O_Ring_Seal_p/470-000047261.htm
Local Trans parts supply house should have them also.
Few more:
http://www.transpartsonline.com/TPOTop/pp.asp?Image=NI&Number=13580A&Description=DETENT CABLE SLEEVE 350 78-86&Addition=null&Price=0.98&Pic=350&Variable=25
http://www.transpartsonline.com/TPOTop/pp.asp?Image=NI&Number=13925&Description=MULTI LIP SLEEVE OIL FILLER TUBE GM&Addition=null&Price=0.98&Pic=350&Variable=25
http://www.transpartsonline.com/TPOTop/pp.asp?Image=NI&Number=13580&Description=O'RING DETENT CABLE 350 69-78&Addition=null&Price=0.7&Pic=350&Variable=25
http://www.transpartsonline.com/TPOTop/pp.asp?Image=NI&Number=3291L&Description=O'RING FILLER TUBE VARIOUS GM/ FORD/ CHRYSLER&Addition=null&Price=1.82952&Pic=350&Variable=25