You are correct that the cultivar 'Niagara' is a white (green) grape that has seeds. It is much like Concord and has a similar "foxy" taste and aroma. In fact, one of its parents is 'Concord', and 'Niagara' is often referred to as 'White Concord' as a result. You can learn more about 'Niagara' on page 88 of the guide produced by Iowa State Extension:

A Review of Cold Climate Grape Cultivars

It may be possible that the vine you you planted is a 'Niagara Seedless'. It was formed as a mutation of 'Niagara' that was seedless. 

'Niagara' is a common grape sold at garden centers and to home gardeners. If you want the seeded version, you will want to reach out to local garden centers to see if they carry it and/or can order the vines in. 

Since you are in Oregon, you may find that online vendors (i.e., seed and plant catalogs) will not ship to Oregon because they must comply with the Oregon Grape Quarantine.  Some vendors are not willing/able to comply with the requirements of the quarantine.