If you do decide to use vintage sewing notions, visit yard sales and flea markets. They'll often have things you wouldn't have thought of. But make sure to check the condition of each item, especially at yard sales. Sometimes the item you see looks good but is really unusable. Ask questions, and don't be afraid to walk away and look elsewhere. You can also check online. Ebay sellers usually have something worth buying, so check weekly to find what you need.
Many vintage patterns call for fabric-covered buttons or belts in their construction. In this case, you can probably use modern notions. Most fabric stores offer these items. If they don't, they can usually order them or at least help you find a retailer who does carry them. Once applied, a modern fabric-covered button looks much like a vintage one, so don't worry about these items too much.
One of the questions I'm asked about frequently is the use of hook-and-eye or snap closures in vintage patterns. Back in the day, these were the closures available to anyone wishing to make their own clothes. They didn't have zippers. If they did, they weren't readily available and came in limited sizes. We have more variety when making clothing today. It's not difficult to use an invisible zipper in a vintage pattern. They can usually be stitched in place of the closure the pattern indicates. Of course, if you're hoping to create an authentic vintage garment, you'll have to use vintage closures.
The choice to use vintage sewing notions is yours. They may be a little more difficult to work with, but the result is well worth the effort.