Looking for yarn alternatives.... How do you go about it?

Hey everyone...
I’ve found another really lovely pattern for a summery shirt... But... as is so often the case, I can’t get the specified yarn here! So it’s only available online, and with €25 shipping (international)...
  When you have to find a yarn alternative, what do you base your choice on?
Yarn length, material, gauge, needle size???
The original is a blend of cashmere and cotton...
  Unfortunately, I don’t have anything comparable to “know” how the knitted fabric will behave—how soft it is, how it drapes… etc.—so it’s really not easy for me to find the right yarn…
How would you go about finding the “right” yarn?

I look at needle size and yardage.

The yardage doesn’t have to match 100%.
I calculate the total yardage needed for the project and then figure out how much of the alternative yarn I’d need.

I’d look for a yarn made of the same material and check the length. I’d browse manufacturers’ websites to see if there’s a similar yarn.

Maybe there’s a knitting group online that exclusively uses this brand—I’d look for or ask about a place to share experiences there. 

By yardage and gauge. Because the gauge really has to be just right. The best thing would be to pick out a similar one at a local store. That way, you can actually feel it. 

The problem here is that there aren’t really any “local” stores.... Either I’m a total “knitting nerd” because I know exactly what I want, or they just aren’t “knowledgeable” enough. ... I’ve already asked about an alternative there—"We don’t have anything like that"—which I already knew, since they obviously didn’t have *that* specific yarn... but they couldn’t even show me or suggest a single alternative.
I’ll probably go back anyway; I’ll take a photo of the yarn along with the description beforehand and browse through their selection....

If necessary, you can also convert the gauge—if you know a little bit of the rule of three. (Yeah, who would’ve thought back in school that you’d ever need that again? 🙂). But I guess the feel—how soft it is—is also important to you. The cashmere makes the cotton softer, after all. Maybe a cotton-acrylic blend or a cotton-bamboo blend would work well.

I look at the needle size; then the rest usually works out more or less. If the needle size is the same, the yardage can’t vary that much. And then I pick a material that feels right to me—I really like cotton-bamboo, for example.
Sure, then I do a gauge swatch and do a little math, and it turns out perfect.
There aren’t any stores around here, so I always end up picking yarn online pretty much blindly, but I haven’t been disappointed yet. You kind of have a sense of how different blends might feel.
But that’s exactly why I always love it when the patterns here include alternative yarn suggestions—great service!

Cotton-bamboo sounds good 😍👍
....and doing the math is also helpful. This happens to me pretty often, since I frequently change the pattern in a project, and then with increases and decreases, the whole thing still has to look balanced...
  This shirt features a mix of patterns, ties in the back, and has a slight stand-up collar. I think it’s really important how the yarn drapes after washing...

@Veronika...... I often order my yarn “blind,” and thanks to the selection in the yarn swap package, there’s one yarn I won’t be ordering...🙈 I actually had a yarn on my wish list, but then I saw it in person and didn’t think it was that nice after all... Otherwise, I have “my” dyers—I’m totally into hand-dyed yarn in 400m/100g.... 

Yeah, I also have my go-to suppliers for hand-dyed yarn. Most recently, I got some sock yarn with bamboo—heavenly soft, absolutely wonderful.