Well, are you buying a pair or some (i.e. more than one pair)?
I might use 'some' for a single pair as well as more than one pair.
Badly-set question. Both are fine.
Some does not sound natural
Yes, it doesn't sound natural. Some could mean a number of trousers. what if the number is an odd number when trousers comes in pairs like scissors. You can't cut something with a single blade.
I think "some" is used when referring to an indefinite number of objects. We can say, for example, "I need to get some change". I don't really know the exact number of coins that I need, I just need enough to buy something.
In case if I need to buy more than one pair of trousers, I would not say "some", but "two pairs of trousers" or "three pairs...".
You should never say "some" in this context. As 'gardemarine' suggests, one might say, "two pairs of". To extend this though, one might also say, "some pairs of trousers".. or simply, "trousers" .. which is a bit more vague.
'some' sounds better to me.
well, I'm confident to use the word "pair".
Some trousers is absolutely fine. As an earlier poster said, it relates to an unknown number of trousers. If I know that I am going to buy two pairs and only two pairs, that would be the only time where I would specify a number of pairs. Even for a single pair I may say "I'm going to buy some work trousers this weekend". That is also absolutely acceptable and is, to me, interchangeable with "a pair of trousers". Pair relates to plural so therefore "some" makes just as much sense.