Yeah, 'decent price' is very specific isn't it? LOL...
As for Marathon, unfortunately they don't make them for the Montero.
IMHO, if the covers don't fit well, you may was well buy generic covers instead from any car parts place.
http://www.wetokole.com &
http://www.seatcoversunlimited.com are two companies that make Montero specific seat covers for both the front and rear seats. Not sure about the 3rd row though. Never used either of them although some in Phoenix I know have used Seat Covers Unlimited for other vehicles with good results.
Alternatively Escape Gear in S. Africa makes some really nice seat covers for Montero's too (seen them on LC's) but they're not exactly inexpensive for a set. Paul May at Equipt carried the brand a few years back. Not sure if he still is. Pretty sure he'd have to order the covers for you though.
Additionally, if you're putting on covers because your old ones are worn out, then you should really consider re-upholstering them instead since it's very likely the foam is also worn out.