Where can official Sennheiser batteres for HDR_45 wireless headphones

You could try taking out the batteries for a minute.

If that's no help then contact Sennheiser customer support:

Sennheiser Wireless Headphone HDR 120 Instruction Manual
read

If you twist the earpiece,one of them has two NIMH AAA batteries ,worth checking out if these are gone ,which is what happened to me.I was about to bin my rs130's and just stuck two new rechargeables and the earphones are working again!!!!!

As for $20 dollars from sennheiser for replacements!?!?!?
You should pick up two NIMH batteries for a couple of bucks on ebay or amazon.DO NOT USE ORDINARY BATTERIES!!!!!

assuming the battery is charged, have you tried to switch to a different frequency band, etc.?
check page 28 of the user manual for possible actions when there is no sound in the headphone.

http://www.jntworld.com/catalog/index.php?cPath=21_1002&currency=gbp&osCsid=c5b2f0de48c59564afb36dfebba513ac#tabbed_box_1

Here you will find something.

You can download the manual for the product from the below link:

http://www.wireless-headphones.net/rs-45_manual.pdf

Hope that allows you to resolve the issue. Please revert back for any further assistance.

Regards

Anand

Check out My tip on headphones, at the bottom of it you will find a link to a place that should have them. Just click my user name to find the tip.

A replacement battery can be purchased from Sennheiser for $20.00.
Battery Part Number: BA 151
Sennheiser Link:
http://www.sennheiserusa.com/newsite/productdetail.asp?transid=004146

A replacement battery can be purchased from Sennheiser for $20.00.

Battery is: BA 151

Sennheiser Link:
http://www.sennheiserusa.com/newsite/productdetail.asp?transid=004146

Did you try to remove the batteries and at the same time unplug the power of the base for 30 sec ?