This is a bit extreme, but if you can afford it and have a GOOD center near you, a stay at a hospital might be worth considering. If (and I stress the IF, because not all centers are as good as the one I went to) the person in question would benefit from "taking a break" from the world for a while and being in a place where they can examine themselves and their feelings without any outside distractions, a hospital stay can be really helpful. It was for me.

Other than that, it sounds like your mom is still dealing with a lot of grief that she isn't dealing with in a healthy way, and may still need treatment for her bipolar disorder. Buying things spontaneously is, if I remember correctly, a symptom of that. Therapy and medication can be very helpful for both issues, but why can't your mother have that kind of help? Is it a financial issue?

If it is finances, perhaps you should talk to her about not having credit cards any more. She probably won't take to it very well, and is certainly able to apply for more behind your back even if you take them away, but you could always merge your bank accounts so you can see what she's buying, when, etc. It sounds invasive but it might give her some incentive to try and curtail her spending on her own.

Either way, good luck, and kudos to you for caring so much for your mom! She has a truly loving child in you.