For a high-end, curated look, several boutique sites specialize in unused vintage postage:
: Specializes in custom-curated postage sets specifically designed for flat-lay photography and formal invites. Custom Wedding Rubber Stamps buy wedding stamps online
If you are looking for (for return addresses or "Save the Date" branding) rather than postage, consider: Love & Wedding | USPS.com For a high-end, curated look, several boutique sites