Just to clarify, bootloader unlocking (which is required by some custom ROMs) is not the same as SIM unlocking. SIM unlocking is not generally done by ROM's, you must pay someone to use your IMEI number to unlock your phone for use on other networks.
Edit: However some phones do have codes you can find online that will allow you to unlock the device for other networks. Some versions of the Ace can be unlocked using ADB as well. When searching for information on unlocking and or anything related to the phone really, use the model number rather than just Samsung Galaxy Ace as there are several version. The model number can usually be found under the battery.
One more Edit: In case you need a place to do it, http://cellphoneunlocker.com/
is a place that will do it. I've never used them personally but they are recommended frequently for Canadian users. Couldn't tell you if they're good, priced well or effective though.