Strut casings?
When you replace your struts, you are actually just replacing the strut inserts. When you remove your old struts and springs, you will remove the old insert from your existing strut member.
If this is not what you mean, please give more details. Your picture didn't show up on my end.
If you are talking about going with a coilover setup, the part you are talking about slides over your existing strut tube, and allows adjustment of the spring height.
As far as where to get your new Tokico struts, many places sell them. Don't know who currently has the best deal on them. I used KYBs, and have been very happy. I don't spend much time on the track with mine.