I made my Vriska fangs out of white Polymer clay (AKA Sculpey). I pushed them onto my teeth and shaped them, then baked them for 15 minutes at 250 degrees. You need denture adhesive cream to make these stay on as well. Honestly, they look better than the ones I bought for Nepeta so I would suggest doing this method.
Here is a fantastic tutorial showing a different method of making your own fangs.
I got my Nepeta fangs from Party City because I didn’t have time to make them. They’re called custom fit vampire fangs. Instead of being a full set of teeth, they’re two individual teeth made out of plastic with thermoplastic in the center. You heat up the thermoplastic with hot water and mold them to your teeth. Although they do fit very nice, you have to get denture adhesive cream for them to stay in. This will make them stay in all day. You can get this at CVS or Wallgreens.
A lot of cosplayers have suggested and these for several trolls. They have ones that would look good for Terezi and Karkat, Nepeta and Vriska, Sollux, and Gamzee. Basically any troll.
Here is a good video tutorial on Teeth painting, which gives the effect of fangs without having to buy or make any. You can also use this method to do John and Jade’s buck teeth.