You cannot get the $ back from the Canadians but you can usually get them back as a $ for $ credit on your US taxes.
If $600 or less and married just take them off at the foriegn tax credit line on 1040. If above $600 you will need to file IRS 1116 and if your marginal tax bracket is above 15% you usually will have no problem getting full credit due to a ratio test in the form. If 15% or lower you may not be able to credit all the $. They can be carried forward for potential future use.
Nothing, if you hold in an IRA it is gone. If you hold in a taxable account you deduct the tax on your annual federal tax form under the category 'foreign tax withheld' as a direct credit against your tax bill. The limit is $300.00 single or $600.00 filing jointly per year. C