There exist several attempts to implement graphical and/or vendor-independent tools for pulse-sequence development - these projects might be a potential source of pulse sequences as well. I've just checked ODIN and SequenceTree (see links below), where several sequences seem to be available.
Some more (similar) approaches are mentioned in a recent article about "SequenceTree" (referenced below).
http://od1n.sourceforge.net/
http://od1n.sourceforge.net/sequences.html
http://wisdmhub.org/dev2/apps/sequencetree/
https://github.com/magland/sequencetree4
Article Pulse Sequence Programming in a Dynamic Visual Environment: ...
Agilent has a website to support it's MR user base (https://spinsights.chem.agilent.com). This site contains a User Library where users can upload sequences. There are a few imaging sequences there, but not many. You need to create an account to access this site.
Bruker has recently started it own User Library (https://www.bruker.com/ppg). This site includes a specific section for imaging sequences, but no images sequences have been uploaded to the site yet. There are a number of spectroscopy sequences available. You also need to create an account to access this site.
The vendors of clinical MRI systems, generally consider their pulse sequence source codes to be proprietary information. Even getting access to their programming environments normally requires signing nondisclosure agreements. I think in most cases this would prevent the posting of even user developed sequences to a public repository.