The i1000 does not have a SIM- you'll have to contact NExtel.

The i1000 Plus is an Old-School Nextel Phone which means you won't be
able to affix ANY SIM CARD whatsoever on ANY old-school Nextel Phone.
Other than areas that are willing to sell old-school Nextel Phones over
the Internet, you won't be able to use any SIMs on those.  Old School
Nextel Phones does not take advantage of the SIM Card Technology putting
you at a disadvantage there.

More likely you'll have to contact Nextel.  I highly recommend that you
upgrade to a Nextel Phone which takes advantage of the latest version of
the SIM Card Technology.  The i205 should be a reasonable Nextel Phone
if you can't live without the Candy-Bar Schemes.  I really don't
recommend going to any old-school Nextel Phones, though as the i1000
uses an outdated version of SIM Technology, so you'll have to contact
Nextel before trying your luck.