Search the gunshows and gunshops for a used 336 Texan or other straight stocked Marlin. They are out there if you look around. I prefer the straight stocks and have a 60's vintage 336 RC in .35, a 336 Marauder in .35, and a 336 Texan 30/30, all with straight stocks picked up at gunshows and shops.
Perhaps if you bought a new pistol gripped 336 and sent it to Marlin, they could possibly restock it with the straight stock which is currently used on the 336 Cowboy. I have read on this forum where others have had 336's made into "Cowboys" by Marlin where the 38/55 octagon barrel was added. Don't see why they couldn't furnish the stock and lower tang. Might be worth a call to them to find out if this was possible.
Also, changing the stock and lower tang isn't all that hard to do. You could buy the parts and do it yourself.
f you have a straight stocked 336 take a look at the reference forum it tells you what to switch to change from straight to pistol grip or from pistol grip to straight. Just take one of each and make the switch.
All of the above are great suggestions and will get you a straight grip Marlin, however there if you are not in a hurry there are original striaght grips available if you are patient.
I waited a year or so for a striaght grip .35 RC of 1957 vintage, it was well worth the wait. It is my favorite rifle to just look at. I will not put a scope on this one because it would compromise its classic lines. I paid $240 for it but since then have seen them sold for upward of $400, however that's a bit extreme IMO.
If you really want one, try shum8's link, the price is a little much but if you are looking for one for Christmas, and want an original, heck why not??
There are probably a few NIB 336 Cowboys in 30-30 out there. I've seen a couple listed on the SASS Wire recently. Otherwise, used 336's with straight grips aren't that hard to find on Gunbroker or Auctionarms. Where do you live? I've got a Marlin 30TK that I have upgraded to walnut. It's got a half-magazine and an 18.5" barrel. I would let it go for $225 + shipping, unless you are from NC and we could do a FTF deal. I've got a couple pictures I could send.