As a general thing, I'd say that most 15mm goblins tend to be a bit too well armed and equipped to make good snotlings. They also have much smaller heads as a rule. Although most goblins have big heads, 28mm snotlings have proportionately
huge heads.
One reasonable fit would be the Caesar 1/72 goblins (if you can find them!). They've got slightly smaller heads and are a bit taller than some snotlings, but they've got the right 'vibe' (pictures from Plastic Soldier Review):


If those prove hard to come by, you could look at
these guys from Magister Militum. They're a bit smaller than snotlings, but are probably the most appropriate of the 15mm ranges. You could mix them in with
some trolls from the same range for a bit of variety (although the trolls will be goblin-sized).
Otherwise, you could look at the Blighthaven/Demonworld goblins from RPE, but they'll be a bit small:

Another reasonable bet might be the "trollbins" (lesser trolls) from RPE. They've got the right look and should be a bit more snotling-like in size, but have only limited poses:

You can see how the trollbin (at the back, to the right of the full troll) compares with the RPE goblins here. I'd say the trollbin is roughly the size of a snotling, and the goblins are a lot smaller:

Finally, Alternative Armies does these lesser goblins, which are between 13mm and 16mm in height, apparently, and have the advantage of being by Kev Adams, even if they're a bit well armed for snotlings:
