如何在NCBI上查找质粒序列，比如pET28？

Vector Database For addgene
http://www.addgene.org/pgvec1?f=v&cmd=listvecinfo

I think this website is what you want. In order to query out the pET28 squence in NCBI, you can use blast tool in NCBI with the similar vector sequence from the vector site.

I query it for you with blast and get many results. Maybe, there is some one for you. The id to retieve the result is JYGFES63016.

The following is the part of the results:
EF442785.1 Expression vector pET28a-LIC, complete sequence 9450 9780 98% 0.0 99%
EF456735.1 Expression vector pET28-MHL, complete sequence 9376 9706 98% 0.0 99%