The size of the PubMed database makes it a challenge to search for research instruments by topic. Like other databases, it does have fields you can search, one of which is MeSH (Medical Subject Heading). There are a few MeSH terms that may help with locating instruments. These include:
MeSH is probably the most effective way to search PubMed for instruments by topic.
If you know the name of the instrument, try searching for "instrument title" being sure to enclose the name in quotes.
Due to the size of the database, and in particular because of the way information is entered into PubMed, this is probably not the best database in which to find instruments.