The XB24-ACI-001 is an XBee Series 1 with a chip antenna, which as you've pointed out has been discontinued.
The first and last part of the model number, highlighted in red, indicates it is an XBee Series 1 with 802.15.4 firmware, non-DigiMesh.
The second part of the model number, highlighted in blue, indicates the antenna type, in this case a chip antenna.
XB24-ACI-001
Therefore, if you can find an alternate XB24-XXX-001 in your area, they differ only in the type of antenna.
Some possibly alternatives are:
XB24-AWI-001 Wire Antenna
XB24-API-001 PCB Antenna
XB24-ASI-001 RPSMA RF Connector for External Antenna
XB24-WFUIT-001 U.FL RF Connector for External Antenna
You can also substitute any of the XBee Pro Series 1 units for a non-Pro XBee Series 1, the first part of the model number will be XBP24 instead of the XB24
XBP24-AWI-001 XBee Pro Series 1 with Wire Antenna
All of the following specs apply equally to the XBee Pro Series 1 models.
Note:
Stay away from model numbers with a Z within first or second part which indicates the unit is an XBee series 2 with ZigBee firmware, which are NOT compatible with series 1.
The same is true of the 004 or 003 sequence in the last part of the model number.
XB24-Z7PIT-004
The sequence 250 in the last part of the model number indicates an XBee series 1 with DigiMesh firmware and although they can be typically reprogrammed with the non-DigiMesh, 802.15.4 firmware you probably should avoid them.
XB24-DMWIT-250
Also make sure the XBees are 2.4GHz units and not one of the frequency models, like 900MHz, etc.
I would normally opt for a unit with an external antenna connector, as utilizing an external antenna can usually increase the range of the device significantly.
BigDog