If I happen to see staff I normally only see in uniform in their own clothes it gives me a thrill.
I get a bit of a crush if I catch sight of staff I normally only see in uniform in their own clothes.
偶々見る・目に入る・見かける ＝ happen to see; catch sight of
いつも制服姿の店員さん = (store) staff who are always in uniform
見るときはいつも制服姿の店員さん = (store) staff I normally only see in uniform
萌える ＝ have a crush; be infatuated (この文脈で「gives me a thrill」でも良いと思います。
私服で ＝ in （their） own clothes
アメリカ合衆国 
