I don't know if you need new or not but I would consider ebay Drueke made a few nice sized ones and the seller chess bazaar I don't know if they offer mag sets or not?
Need help picking good chess set
I have never bought chess stuff except books from Amazon. But if it's sold by Amazon and you don't like it send it back easily.
In Amazon, do a search on, "12" magnetic chess set".
There you will see a lot of choices.
If you want to spend less the plastic ones are there.
The wooden ones are more expensive but very nice.
Just make sure you get one that is marked as Prime and you'll be fine.
Also look at the customer ratings and reviews. Those are good.
I want a 12 in magnetic chess set to be ordered from Amazon. My cousin has.Prime so no postage. Please help me pick out a good one from a reliable company.