Here's one on eBay with a very similar correct seal:
http://www.ebay.com/itm/Magnetic-Oil...-/350396106072
If you find someone that sells "The DIMPLE" brand drain plugs they would have it also. I believe the brand originated in the UK. JJ of MagDana I thought carried that product to a new business but I haven't communicated with him for maybe 4 years.
There are motorcycle shops that do "The DIMPLE" also but you need to watch the 5/8-18 from them, it's a very old different style HD drain plug. I've a friend that has a couple of them
Try some searches using "The DIMPLE" branding - that might work also. Haven't tried.
Here's a link to maybe a "distributors page" for The DIMPLE:
http://www.drainplugmagnets.com/authorizeddealers.php
The eBay offering is really inexpensive.
You might stumble across a couple of my posts in your searches and I did mention that the OE/GM plug has a sufficient "counter-bore" that you could insert a magnet of your choice. As strong as you can find!