The part you are looking at is a seatpost binder bolt. These are easily available at a variety of price levels and different materials, generally chrome-plated cromoly are the best but the stainless steel ones are quite nice.
There are a couple of different types; the most common are the type that have a little nub that fits into a slot/cut in the hole for the binder bolt, but not all frames have this. Another type has a serration to fit a frame with round holes only. To me it looks like your frame can use the first type.
Sizing is generally standard but sometimes these are made overlength and need cutting down to size. In a pinch, you can use a suitable length recessed brake nut and an M6 bot with washer to get a good clamping action.
These bolts used to be manufactured by big names like Campagnolo etc etc but the only branded new ones I've seen recently are Tange. They are generally nowadays otherwise unmarked by branding.
When you know the proper name of the part, it is easy to search your favourite bike parts supplier, marketplace or speak to a bike shop over the phone.