The great thing about Univers is that it comes in so many weights and widths. It's a real powerhouse of a typeface.
Very few typefaces come with the range of fonts that Univers does. So there's really no universal 'alternative'.
Now, if you are only using one particular weight of Univers, we could maybe find a font with similar metrics.
All that said, you don't need an alternative. You can license Univers as a web font:
I would consider the google-font "Oswald" It comes only in 3 weights and has no italic or oblique version, but reminds me of the Univers Condensed type. Try changing Oswald 1pt less than Univers, regular -> light and bold -> regular.. Last, change tracking to +50..
That should do the job.. :-)..