I would think the Celery seeds would be cheaper and goes a lot further. You would need a blender or better yet a coffee grinder to get it more in powdered form. I would think you would want to make the tea fresh each time, but maybe grind some seeds for the day, then refrigerate so it doesn't go rancid. Ideally if you could invest some time into making a fresh groud batch each time for this small amount of time, that's what I would do.
I don't know where you live, but perhaps wildoats.com has a store in your area, or if your store orders bulk herbs/spices see if they have a local source. May have to order then online which is more expensive, but maybe a relative can send it to you. Hopefully you will find it your neck of the woods. Keep us posted and I'm rooting for you. I would also look to clean up my diet, as I am not sure what you eat, but I do a predominately raw-vegan diet. However, it's my daughter who had the problems and not me and she eats animal products, etc. Maybe even some dairy. Take care. Oh, my daughter didn't pass any stones.