Decisive strike is only available through the monk-class ACF in Player’s Handbook II. Nothing else is all that similar, even.
There are options for getting flurry of blows as a non-monk, though the wording on some of them is ambiguous.
|“gain […] as if monk […] monk level +”
|Disciple of the eye
|“Flurry of Blows (Ex): [non-monk]”
|Races of the Dragon
|Monk of the enabled hand
|“gains […] as if monk […] monk level +”
|Sun soul monk
|“levels stack […] for purposes of determining”
|City of Splendors: Waterdeep
|“levels stack […] to determine”
|Secrets of Sarlona
So disciple of the eye straight up has a class feature called “Flurry of Blows (Ex),” and it explicitly discusses how that works for non-monks. That’s the gold standard right there.
Arcanopath monk and monk of the enabled hand both use the word “gains” in reference to flurry of blows, as part of larger class features that explain getting and/or progressing various monk class features. These both seem explicit and clear to me.
Less explicit and clear, sun soul monk and Tashalatora both say that monk levels and certain non-monk levels stack to determine flurry of blows. Nothing says that monk levels can’t be 0 in either case, which is fairly-widely accepted as allowing non-monks to gain flurry of blows, but the rules are less explicit and it doesn’t sit well with some DMs.
As evidence for sun soul monk and Tashalatora, though, I would point to the serene guardian from Shattered Gates of Slaughtergarde. The serene guardian explicitly gives non-monks bonus feats instead of progressing monk features, so it isn’t an answer here, but for monks, when it does progress those features, it explicitly progresses “flurry of blows attack bonus,” not flurry of blows as a whole. This is evidence for things like sun soul monk and Tashalatora, which do progress flurry of blows wholesale, allowing non-monks to access flurry of blows.
Now then, in the case of a monk who has taken decisive strike, we may have a problem. All of these are say you get flurry of blows “as a monk” would, or that your levels “stack with monk levels” for flurry of blows. Except your monk class doesn’t give you flurry of blows if you have taken decisive strike. It may not really matter—after all, most of them refer to “a monk” or “monk levels” rather than “your monk levels” or that “you get as a monk” or something. So maybe they refer to some generic monk, one that presumably didn’t take decisive strike. This is a totally reasonable interpretation of the text—but the whole situation is on very iffy footing because there just isn’t any explicit handling of the possibility.
Even if we acknowledge this as a fatal flaw in the whole plan, the disciple of the eye still gives you a class feature called “flurry of blows,” so for prerequisites rather than using the thing, it’s sufficient even if we rule it does nothing for you.
But really, if there is a problem, it’s purely authorial error and oversight. Alternate class features as a concept didn’t show up in all that many books, they weren’t something that authors wrote around. The rules for ACFs themselves don’t really address the problem, either, though they should. Originally, they were a variant idea in Unearthed Arcana, which was full of variant ideas that really needed a bit more DM adjudication and adaptation than your typical sourcebook. But Player’s Handbook II didn’t do that when it used the concept, which I think is a failing. Even though Player’s Handbook II as a sourcebook “should be” just usable “as is,” I think in this regard it isn’t, and a DM should work with players on these kinds of issues. Decisive strike should just count as flurry of blows for prerequisites in the first place, really. But if it doesn’t, then your monk levels shouldn’t inhibit you from getting flurry of blows from things that would ordinarily give it to non-monks (or stack with the existing one that monks are assumed to have). But nothing of this situation is clarified by the rules, so you will have to ask your DM about that.
Anyway, for the record, Tashalatora is by a large margin the best option of these. Of the more explicit cases, disciple of the eye is by far the easiest to enter—it just requires Improved Unarmed Strike, Concentration and Spot at 8 ranks, and the dragonblood subtype (which is available from many races or by taking the Dragontouched feat). Both arcanopath monk and monk of the enabled hand require three weak feats (Dodge, Mobility, Deflect Arrows or Combat Expertise, Improved Disarm, Deflect Arrows) in addition to Improved Unarmed Strike and some other minor requirements. Disciple of the eye also definitely has the strongest class features of the three, plus it has that benefit of being something that works for prereqs absolutely no matter what.