Hello:
I am a professional genealogist and I specialize in Basque Research. I am not, however, soliciting business.
I just saw your post and wanted you to know that yes, LANDAZABAL is quite a common surname in the Bilbao area and in other towns/cities in Vizcaya.
In Bilbao there are at least 9 parishes with microfilmed records and so it would quite an undertaking to try to find your Great Grandmother without knowing more about from which part of Bilbao she came. If she was born post 1870 I would suggest contacting the Civil Registration office in Bilbao to see if they have a record for her. Sometimes in the big cities they are not so helpful in sending information (especially if your information is sketchy) but it is worth a try. Often this information will lead you to the correct parish and then you can consult the microfilmed records.
Best Wishes to you in your search!
Happy 2008.
Susan Ybarra