I am not sure if you can buy insulin online - but worth looking into as it will be cheaper online. Your vet is obliged to provide you with a prescription.
Defibately invest in a glucometer for curves as it means you can keep a much closer eye on things. It costs about £100 for a curve at the vets and also cats are stressed in the vets and this puts their blood glucose up.
The most important thing with a diabetic cat is consistency with diet. There are a few good diebetic management diets out there. Bith with dry and wet varieties. These are good because a) the fibre levels mean a slower release of energy and more importantly the formulation wont change, unlike your normal cat foods that do change their ingredients periodically.
Do expect the next few months to be a bit up and down in terms of stabilising blood glucose. It takes time to get the insulin dose so you will start on a low dose and see how she responds. So even though it is all new to you do invest in a glucometer soon. Its better to use the one used by your vet for consistency. Most are really easy to use - this will save a fortune in curves as you will need them more frequently until she is stable.
Once stable we have cats that we curve every six months and they tootle on fairly well.
Your vet nurse will be more than happy to answer questions and will explain everything to you.
Catteries will take STABLE diabetics but if you need to go away in the meantime your vet may offer boarding although this will be considerably more expensive and unlikely to be covered by insurance.
Your vet will tell you what to look out for in terms of low blood sugar and probably advise you to keep some glucose gel in stock jyst in case.
Im not going to lie - it isnt easy at first but once it has become part of your cats routine she can live a happy and healthy life.