Go up to the top of the forums list, go into the New LS1 owners-newbie tech forum, on the top of that page there is a part number request. Be sure to list manual or automatic trans, even though it's in your signature no one knows if you're working on that car.
I checked gmpartsdirect.com, and gmparts-direct.com, they don't list part numbers.
I didn't know it before but there are two different listings for motor mounts for '98-'99 depending on transmission type.
I see you are not in the US so I guess finding parts may be difficult. The local parts supplies here have to order them also.