In this link, it is explained that shimeru is a transitive verb and so it can take the -te aru ending but shimaru is an intransitive verb so it can take the -te imasu ending.
However, in some grammar textbooks (such as this one here) I have seen shimatte arimasu as well:
Tsukawanai mono wa kurōzetto ni shimatte arimasu.
I leave things that I do not use in my closet.
What is the difference?