Hi Mother of Murray,
There are some sources of mattress components and layers including mattress covers listed in post #4 here.
Before you begin with a complete DIY mattress design I would make sure you have read some of the posts on the forum about this such as post #15 here and post #5 here and post #7 here so you have realistic expectations. It can be a lot of fun and be a good learning experience but if your main reason for going in this direction is to save money then it may not end up being as cost effective as you believe and choosing some of the “kit” mattresses that are available where you can also choose your own layers may end up being more cost effective and successful (and probably less time consuming).
Phoenix