Problem solved. . .Found the OBX gasket @ 'Tuning Depot' (eBay seller of OBX products). Their eBay listing didn't mention that the Civic / Prelude (??) downpipe gasket also was exactly the same as the one OBX uses for Accord F23 applications, too. After confirming w/ them that it was the right gasket, they shipped it from the left coast to GA in 2 days.

In fact, I'm keeping that gasket as a spare since the orig OBX gasket was in okay condition - esp after using Permatex high-temp silicone (copper) sealant, along w/ fixing the dumb-ass problem caused by the exhaust shop that re-welded the cracked welds, and then re-installed the header w/out using a washer under the nuts (for the 3 bolts on the flange). The raised 'boss' between the bolt head and where the threads start was exposed (w/out the washers) as it exited the nut side of the flange, making it impossible to tighten down the nut sufficiently to get a tight seal on the gasket. (note: such bolts are often larger in diameter on the portion of the shank w/out threads) I found a super exhaust shop ('Jasper Muffler' - formerly known as 'Cherokee Muffler' in Canton), but now in Talking Rock, GA - just north of Cherokee County - that solved the stupid problem in 10 minutes for $10. Such a deal!

Thought I'd pass this along too, as it's often a simple, human-induced problem that causes all sorts of unnecessary time-consuming & often not needed expensive fixes. The 'problem' all began when I took the lazy way out of re-installing the header after it had been re-welded (rather than doing it myself), as I had a a different repair shop w/ it's ASE trained mechanics (sic) re-install it, and they caused the problem, but were unable to find a suitable gasket, sending me off to find the gasket I didn't need . . . If I had done the work, I would always use new washers (and also would replace the bolts and/or nuts if they were gawled, or otherwise damaged), as I am a firm believer that mundane hardware - esp the cheap shit made in China - is good for just so many re-tightenings, esp when heat or any physical stress is placed on such connections. (bolts stretch, distort, threads get damaged, bolt heads get rounded-over, corrode, and even weaken from 100s of extreme temp-recyclings, and so on)

Also, I suggest that one make a tracing/template of any non-OEM gaskets, as one cannot rely on most aftermarket suppliers to offer any after-the-sale support, as OBX demonstrated in this case. That little drawing may make it much simpler to find a suitable gasket (or make one) before tearing a car apart to replace that gasket, as almost all parts houses these days only look up a part number on a computer, and most don't have a clue when it comes to sourcing replacement parts from Asian aftermarket component manufacturers. Just tape those drawings, plus any pertinent cross-referancing, etc info, in your shop manual. . .But, one would think that I would have followed my own advise when I had originally installed this particular header & catback system 6 yrs ago, esp considering that for almost 50 years I've been messing around w/ building '30s-'40s hotrods, '50s customs, modifying sports cars, & Honda/Acuras more recently - but no - I didn't make that little drawing of that gasket, screwing-up this time, for sure!

"Old farts don't learn new tricks - they just seem to forget what they used to know. . ."