Most shop who sell contact lenses stored colored contact lenses, of course, including red contact lenses. You as an seller in local shop nearby you. If you can't find it. you can also order a pair from online shop. I just search it in Google, And i find several sites selling red contact lenses. They are https://www.youknowit.com/online-shop/red-contact-lenses.cfm, https://www.colorlens4less.com/ and https://www.demonsfx.com/acatalog/Red_Contact_Lenses.html. Hope this helps.
You can go to your local optical stores which offer contact lenses to have a look. There you may find some red contact lenses for cosplay. Before you get them, you are required to have an eye exam first, so the doctor will know whether you are suitable for these colored contact lenses. If they don't offer red contact lenses, you can ask for a copy of your prescription and go to other optical stores to have a look. Or you can search them online and you will find many sites have red contact lenses sold. I will recommend you some like demonsfx.com and youknowit.com.