Are custodians limited to companies with offices in the U.S.?

The IRS publishes a list of approved custodians. You can also ask the custodian for a copy of its approval as custodian from the IRS.

TD Waterhouse - example of a firm that has Canadian connections but also operations in the US. They do offer IRAs. But, you are not likely to find a Canadian brokerage company that only does business in Canada offering IRAs.

I am not sure why you want a Canadian brokerage. You can own lots of different investments that have a Canadian connection within an IRA held at a USA brokerage. For example, I have shares of Norwegian, Canadian and Australian companies (via ADRs) in my Roth. If your question is related to the kinds of investments you can hold, ask your current custodian.