1. MetaTrader Online Market https://www.mql5.com/en/market
For MetaTrader users, the mql5 market is the safest place I know where you can buy a forex robot from.
There are simple forex trading robots that cost as low as $100 and there are more advanced forex robots that execute complex strategies but cost as high as $600 and beyond.
Before you buy a forex robot, you should read reviews about the particular robot to know if others agree that it is good.
You should also read the overview of the robot to know the minimum requirements for its use such as forex broker, account type, MT4 or MT5 compatibility, trading strategy to be used, support you will be getting etc.
2. cTrader Online Market https://ctrader.com/bots
If you are a cTrader user, you can also buy your forex robot from the cTrader online markets and some of the robots are free, some cost as low as $60, while some cost as much as $1,000. However, the cTrader market doesn't give you as much information and user reviews about the cBot as you will get on the MetaTrader market.
3. Pay an expert to build one for you
You can also pay app developers found on remote working sites like Upwork to build you a forex robot but they charge around $500 and even higher.