That would be because Quantegy went bust a year or two ago.
I think the nearest equivalent for you is Zonal 840 (for 407), or Zonal 820 (for 406).
The only difference between the two is that 407 (or 840) is thinner and longer playing. e.g. a 10.5" spool of 406 has 2400 feet, a spool of 407 has 3600.
I believe RMGI's equivalent is LPR35, but that stuff is like hen's teeth in the UK.
Studiospares.com seem to have damaged their site listing for mag tape (you have to search for RMGI or it won't show up), but they're very handy for accessories like metal reels, head cleaner and editing stuff like chinagraph pencils, editing razors and splicing tape etc.
Tape-wise, they sell SM911 and SM900. If you can't get the Zonal stuff in time, SM911 should do at a pinch.
For Zonal tape, give Canford Audio a go. Probably best to check if they've got it in stock first, if you're time-sensitive:
http://www.canford.co.uk/Products/86-323-ZONAL-840-105-inch-NAB-hub
http://www.canford.co.uk/Products/86-313-ZONAL-820-105-inch-NAB-hub
...note that the 840 is almost twice the price.
Also, be aware that the 10.5" spools that Canford sell
do not have a reel. They come on just a core. To use them, you must either buy a separate metal spool which you can disassemble and reassemble around the core, or buy one of Canford's plastic reels and very carefully spool the tape onto that. I can talk you through either method it you like, or if you buy a 7" reel, those do come on a plastic spool.
http://www.canford.co.uk/Products/86-322-ZONAL-840-7-inch-reel
http://www.canford.co.uk/Products/86-312-ZONAL-820-7-inch-reel
How long these last depends on the tape speed. If it's mastering, I'm assuming you've got a high-speed A77 running at 15 ips.
At that speed, a 10.5" spool of 820 will give you approximately 31 minutes of time. Strictly it's about 33 minutes, but it's best to leave a gap at the start and end. A 7" reel will give you about 16 minutes.
For the long-play stuff like 407 (or 840), you'll get about 46 minutes (48 total), or 22 minutes on 7" (24 total)
I hope this helps!