Vet, but they need to advise it before you feed it. If you want something long term or not as powerful then try some of the non-bute things on the market like No-Bute etc.
Bute is a Prescription Only Medicine( POM) and must be prescirbed by your veterinary surgeon. Do not be tempted to buy it/try it from anyone other than a vet.
I would be very wary of "Internet Pharmacies" selling Bute after all the scams with wormers online. Plus to get the prescription you will need your vet anyway so it makes sense just to get it from them.