The programming language really does not matter when talking to a JSON-RPC endpoint so use whatever knowledge you already have invested in.
I have not done any python in the last 10 years or so but it has become increasingly popular lately so I assume that would do what you need.
I cannot vouch for it but there is at least this
jsonrpcclient library on the PyPi repository that might help.
It really also depends on your needs. You mentionned they were sending you an XML file that you were using already. You probably have some script reading it I assume for integration.
You could probably just modify that script to use the JSON-RPC api instead assuming they also have and endpoint that can get you a list of all products.
Are you trying to integrate it with your own inventory system? An e-commerce? etc
If you really just need to do an ad-hoc manual query based on random keywords, then you can even just make yourself a silly little local webpage with a few fields and some javascript that will query it for you :)
For testing the API, you can use something like
json-rpc-2.0-shell to query the API directly from a simple command shell.
Have fun!
Feel free to concact me directly as well.
Pascal Charbonneau