Where can I get a blank durable POA form for the state of Minnesota?

Office max and then have it notarized

Windyridge is spot-on. You are best off having all done by attorney with witnesses and notary seal. while you are there also discuss having a will done or having their existing one updated with a codicil if things and beneficiaries have changed.

I would recommend using a local attorney. Your laws and statutes may differ from other states on many issues. Spend a little money and make sure all bases are covered. It's good that your getting this done. Get it written as broadly as possible to cover finances, medical directives, end of life decisions, burial or cremation etc. once the loved one is no longer able to make these decisions the designated POA must have the ability to deal with any and all affairs.

http://www.ag.state.mn.us/PDF/Consumer/PowerOfAtty.pdf