Amazon Drygoods (Predates Amazon.com to which it is not related. It is a long standing historical clothing supply store, it sells new paper and cloth ready-made collars in historical styles, collar buttons and shirt fronts. The pressed paper and laminate , and can usually last through a 3-weekend run of a college play. The collars are made on the original 1860s machinery of the Reversible Collar Company, original maker of Linene collars, a paper-cloth laminate, which Amazon Drygoods purchased in the 1980s when after the R.C. Co. and it’s successor Gibson-Lee went under after over 100 years of business. So, though these are laminate, they are actually some of the most period-correct collars you can get).
Etsy Store: Fogey Unlimited (Sells vintage and new menswear including new linen collars in vintage designs).
Historical Emporium (Reenactor Clothing, lots of cool stuff.)
Etsy Store: Hidden River Finds (Vintage/antique clothing including many early 20th Century collars).
New & Lingwood (UK company that makes menswear including Eton uniforms since 1865, still makes collars).
Premiere Designs Historic Clothing (Victorian/Edwardian historical reenactor supply, including collars.
Darcy Clothing (Has historical style collars both made like original models. and in machine washable stays stiff with no starch models).
Barker Collars (UK Sells formal wear, including bespoke (aka custom-made) collars, and has a starching service.)
Luke Eyres Specializes in making parts for all those old and fun traditional British uniforms for sports (cricket of course) and professions (clerical, legal, military) that still use styles worked out 100+ years ago. As a result, they sell styles of collar that are not available in the usual Victorian/Edwardian reenactor sites in the US. So if you need legal or clerical bands or 19th Century style military collars etc, they have the lot.
USHist.com (Sells new made Victorian collar styles to reenactors).
Vintage Dancer page on “Where to buy detachable collars”, that lists even more places.
Wizner Dry Cleaning NYC Dry Cleaner – Dry Cleaning Specialists in Fine Garments, Rugs, Linens, Theatrical Costume, Mascot Cleaning. Also does Starched Detachable Collars!
Barker Laundry Service (UK. The folks who did Downton Abbey’s Collars.)
Fedora Lounge: Instructions for Home Starching