A couple of posters here got a nausea of their Mule Team; so I would actually recommend a "cosmetical grade", or 100 % borax; just in case they put additives in it. I have bought mine from Mountainroseherb lately, just to have one I know is safe for you in US; well at least I have had no reactions to it; my cats have some in their water.
I am sure there are other companies, maybe even pharmacies that sell this grade.
There is also the option of capsules sold in the health store; just check the ingredients; it is usually sodium borate / tetraborate etc. Same thing; but feels safer to take?.