The most cost effective way to make a salicylic acid product at home is to buy it as a pure powder and mix it with your other components. Pure salicylic acid powders are available online, such as this one from Amazon. Although a liquid form may be easier to utilize, they generally max out at a 30% concentration, are typically low volume (<15 mL) and are much more expensive than the pure powder.
In order to use a salicylic acid powder to make a 2% solution, you simply need to add 2 grams of powder per 100 mL of solution. If you want to make a 1% solution, add 1 gram of powder per 100 mL of solution. If you wish to make a lower volume solution, say 30 mL of a 2% salicylic acid solution, you would mix 0.6 grams of powder per 30 mL.
It is important to wear gloves when handling salicylic acid powder as it is caustic and can cause irritation. In addition, without additives, salicylic acid powder isn't very soluble or stable in solution. It generally should not be stored for long term use and it may be difficult to get all the powder to dissolve if attempting to make a solution (as opposed to a suspension).
Be sure to speak with your vet/doctor regarding the safety of making and using a salicylic acid product at home before doing so. They may have alternate product suggestions.