in my opinion
whatever you do
do not get them at the retailer you mentioned
( last resort on any part.)
the companies CHUBBINIUS mentioned
are the way to go.
original belts are most likely dry rotted
even though they may look good.
a lot of the aftermarket companies
will re-web yours if you send them in.
and they also sell all new units with new retractors
that will fit, but may not look like OEM as far as retractor housings, etc.
I did the 3 point belts for back seat too,
it was easy, like another member said, bolt hole in roof behind headliner.
dave in NC