Who to ask for letters of recommendation when changing fields after PhD? I want to apply to competitive postdoc programs in a different field than the one of my PhD.  It asks for 3 recommendation letters. What to do if none of my collaborators is well-known in the target field/institutes?

I do have some papers "in between fields", but I didn't have enough opportunity to pursue this interest during my PhD (so neither papers nor collaborators are world-recognized).

Except for advisor, should I aim for:


collaborators in my main field (they won't be recognized in the target field, much less - target institution),
or collaborators from the cross-disciplinary paper (they won't be recognized either, but are from the right field),
or the most recognized profs in the target field (with whom I have been talking, but not collaborating)?
The purpose of letters of recommendation is to confirm your research skills and vision.  Since you're changing fields, you really don't have anybody who can speak to your skills in the new field yet, but you may have people who can speak to your skills in your old field and how they think those may be relevant to the new field.

Much more important than your letters of recommendation, however, is establishing contact with the people that you want to work with in your new field.  Postdoc hiring is generally less like grad school and more like seeking a permanent position in academia or industry: you are not likely to be hired for your potential to develop new skills, but rather for your ability to rapidly start contributing in areas of interest to the lab you will join.  

What you want to look for is thus an overlap between the two that will allow you to "pivot", contributing with your old skills while learning new skills. If you're making a complete and total switch (e.g., field botany to femtosecond laser circuits), then this might be pretty hard to find.  More often, however, there is an overlap that can make it beneficial for both you and the sort of lab you want to join.  For example, if your thesis was on mechanical stress models and you want to move into biology, then you might look for people working on anatomical problems where your stress models might be useful to apply.

If at all possible, you should get introductions to people you are interested in working with in person at a conference or electronically by third parties known to both of you.  Any PI is likely to be constantly getting cold-call emails applying for postdocs from dubious applicants, and you want something that will make you be placed into the category of "person worth replying to" rather than "spam folder."  Here is where your conversations with the high-profile professor may be valuable, if that professor is willing to make such introductions.  Your letters of recommendation are then less about making your case than about confirming that you are who you present yourself to be.