Need Query To Select VarChar Rows That Can Convert To Integer

I have a table of zip codes, some with Canadian zips. I'd like to take
a zip code and search for nearby zips. For example:

Dim theZip As Integer = textbox1.text

....Parameter.Add(@ziplow, SqlDbType.Int, 5).Value = (theZip - 10)
....Parameter.Add(@ziphigh, SqlDbType.Int, 5).Value = (theZip + 10)

SELECT * from ZIPCODES where Cast(zip_code as Integer) BETWEEN @lowzip
AND @highzip

Problem is the letters in the Canadian records cannot be cast as
integers for this process. I get this error:

Syntax error converting the varchar value '53151 1' to a column of data
type int.

Is there a SQL query that can exclude if no cast can be made to an
Integer?

Thanks!

Anton