Scotch 206/207 is even worse I believe for SSS. Best stuff for the 388 is 457 and LPR35.
I'd heard 206 was OK, but 226 996 etc were bad. I have some NOS 206, but haven't recorded on it yet...I'll report back my findings...
You can still get NOS 457 directly from Quantegy for about US$28 /reel (1800 feet), also Maxell UD/XL-1 comes up occasionally, but as others have noted, for whatever reason, it has attained some kind of mythic status, and have inflated the price considerably.
I have good luck with UD, (bought in the 80s). You might also consider Quantegy 041 from ReelDealPA, its is white box stuff, meaning it is "factory seconds" and my understanding is it could be either seconds of 407 or 457. You'll find both pro and con opinions on using it and 031 (the 1.5 mil white box) here, but you can't beat the price. (about US$7/reel)
Depending on the sound you're after, there is also Ampex/Quantegy 642, which has slightly lower specs than Maxell. This and the Maxell is not backcoated though.