Singular or plural?

Plural, but I think it would still be a slightly awkward sentence.

Maybe . . .

A variety of powder mixes are available, each with a different composition. Or something similar.

Agree with whiskers -it's an awkward sentence as worded. You know what you are trying to say but it doesn't come across clearly.

boron said:

Various powder mixes with different compositions are available.

Composition or compositions?

Click to expand...

Various powder mixes ... are available. Available modifies mixes, no matter how the rest of the sentence goes. "Different" implies plural in this case, since there is no definite or indefinite marker. If it were "composition", it would read: Various powder mixes with a different composition are available.

However, since there are various ones, this implies compositions different from each other, not just the original, otherwise it would be counter-intuitive to call them various if they applied to a single other composition.

So yes, you would be correct in saying: Various powder mixes with different compositions are available.