I've read over and over on this board and others that a good substitute for the OEM steam pipe seals are Dorman or NAPA oil drain plug seals. I myself took this advice last month when I did a head/cam change and saved a few bucks by buying the NAPA seals. Well, I'd caution everyone against doing that in the future.
I spent the day yanking my engine out and along the way pulled the steam pipe back off. I noticed that they were wet with coolant, but the bolts were still tight. This is rarely a good sign.
Here's a side-by side of the NAPA oil-drain seal after 400 miles of use (on the left), and my original steam-pipe seal after 125,000 miles of use (on the right)
Notice the frayed edges of the NAPA seal and how its pulling away.
Here's what happens when you apply pressure to the rubber seal. OEM first (yes, my nails are gross. I pulled an engine today. Suck it) :
Still has a nice, tight bond between the washer and the seal.
And now the NAPA seal:
Aaaaand there's my leak. All 4 of the NAPA seals I pulled off were like this. They'd all fallen apart.
Summit Racing has the GM seals,
and I've ordered 4. They've come down a lot in price. Enough to make saving $4-10 not even close to being worth-it.
As with everything, your mileage may vary.
Engine pull picture for good measure: