It is, but if you're buying at the dealer, you'll pay premium prices.
Any auto parts place can order them in for you.

According to your owner's manual, "OEM" is either
NGK part number PFR5L-11, or
Denso PK16PR-L11.

They're pre-gapped, but do check (gently!) to make sure the gaps haven't
got bumped closed in handling.

They're platinum-tipped, for one. That's primarily what causes the high
price.

Also, NGK's "L" designation denotes a particular construction that your
car needs. Incorrect plugs may cause the engine's computer to report
errors.

And before you actually order new ones, check the appearance of the
electrodes on the plugs you have now. Platinum plugs last a long time.
If the electrodes show little sign of rounding, and the platinum dot is
still present on the underside of the ground electrode, you can leave
them in for much longer.

Tighten plugs to 13 ft lbs. The engine cover bolts are ONLY 7 ft lbs!!

You can get NGK or Denso Iridium plugs as specified for the RL for about
$12 each here:

http://catalog.worldpac.com/ibusiness/JSP.v.1.01/wizard.jsp?header=http://www.al
leurasianautoparts.com/header.txt&footer=http://www.alleurasianautoparts.com/foo
ter.txt&partner=ipb&clientid=alleurasianautoparts.com&baseurl=http://www.alleura
sianautoparts.com/&cookieid=25K011QG925K011QG8&year=1999&make=AC&model=RL--001&c
ategory=All&part=Spark+Plug

I would use either NGK or Denso Iridium's without fear.

John