there is also iron on hem tape a hair dryer and something like a 1x board behind the seam and some thing to press down on the front side works too.Is this for a prop you're making, or for a costume (Creature Reacher / Stalkabout style) you'll be wearing?
That, and just HOW oversized? (for 7 foot tall...8 foot tall...more?)
If it's for a prop, you may have good luck making your own - I know it sounds like a pain in the butt, but keep in mind if it's for a prop, you can commit a multitude of sins that won't hurt the end product, unlike something you'd be personally wearing.
(as in: using a stapler to join seams instead of needle and thread. If things are out of line, just jerk it around a bit, and fasten it in place so it looks right. Don't have to worry if it's put together well enough to survive wear and tear of any kind...other than the wind tugging at it. No worries if it's comfortable or not...let alone wearable. And so on...)
Also, if you do make your own, you can put the pieces on the prop after you have it made, and then simply fasten them together (instead of getting the prop together, realizing you haven't dressed it, finding the coat impossible to get on the immobile parts, and having to partially disassemble your prop to get the trenchcoat on).
If you need something that's going to be of actual garment quality, look up some of the big and tall type of clothing stores - keep in mind, they're still going to be tailored to fit "reasonable" sized human beings...anything for someone over 6' 8" or so may need to be custom tailored (I have a couple "rather tall" friends (plenty between 6' and 6'2"or3", but a few are 6'4", two are 6'8", another is 6'9" and one is 7'even) and most of the taller have a hard time with NORMAL clothing, let alone trench coats, suits, and whatnot).