These were the mushrooms that I found. Not all for Poser but could possibly be used.

http://carraralounge.com/index.php?action=tpmod;dl=item62
Carrara mushrooms

http://www.daz3d.com/i/3d-models/0/mushroom-magic?item=10410&cat=&_m=d
Mushroom magic Textures for LIsa's Botanicals A Curious Proliferation.

http://www.daz3d.com/i/3d-models/-/lisa-s-botanicals?item=10408&spmeta=rq&_m=d
LIsa's Botanicals A Curious Proliferation

http://www.renderosity.com/mod/freestuff/details.php?item_id=23576
Morphing Mushroom.

http://www.renderosity.com/mod/freestuff/details.php?item_id=17196
Posable mushroom

There are a few other mushrooms in the Renderosity free area. Just listed the one's that looked closest to what you are asking for.  As mentioned the ones at RDNA.

What you may have to do is find one that is close to what you want and use magnets or create morphs yourself.  Lisa's seemed to be the closest that I found. That or if you are good at drawing out your own. You could use the one sided square with your drawing.

Poser Pro 2010