I live with my parents and kind of in the same situation as you. But I have taken some steps that helps me to wear & wet diapers while also keeping it a secret.
1. At first I used to make my own diapers. I had cut a thik plastic sheet into a shape of a diaper. Used some foam ( Idk the material, it came with glass of some furniture we bought) over it. Layered it with some handerchiefs. And finally stuffed it with lots of tissue. I tightened them on by using safety pins on the plastic. Wore underwear over it. It wasn't very practical to use. The plastic I used wasn't crinkly. So, it was silent. But it was hard to move in.
I had fun using it. It was a daily routine to pee in my self-made diaper before or while in shower. I flushed down the tissue in the toilet. And washed the other equipments. More tissues and handerchiefs inserted, and I was ready to go while the used clothes dried.
Later on I learnt about the T-shirt diaper and used that for some time. But now that I wear real diapers I don't think what I used to make are anywhere near real diapers.
2. If you decide to make makeshift diapers, please don't wet them while you are around your parents. They aren't trustworthy.
3. I have the same problem as you. I can't order online. Besides, there is always a fear of someone could caught me. It took me a lot of courage for me to get diapers for the first time. But now it's easier and I get really excited whenever I go diaper shopping.
The trick is to have a large bag with you. Also, have a safe place for hiding your diapers ready. Maybe go to a store you usually don't shop. I mostly buy baby diapers and it's pretty normal. Even if someone spots you, tell them you were on a errand buy diapers for your baby cousin or something. You probably can't use this excuse if you are buying adult diapers.
4. Trust me. Wearing baby diapers around your parents are wayy safer than wearing makeshift diapers. They are much less noticeable when worn under underwear.
5. If you want to wet while your parents are around, bedwetting may be a popular solution for you. Just make sure to have enough protection.
6. When you go live by yourself, you probably will be wearing more. Baby diapers or makeshift diapers will probably no longer work then. After going 24/7, I need to change diapers more frequently and they also leak.(I still wear baby diapers) . It's really a pain & that's why you are better off with adult diapers.
Extra:
7. No. No. No. As a college student, I find money to be biggest problem of my diaper life.
8. You probably shouldn't tell your parents unless you have medical reasons.