They're sold out at the Sens Store, probably won't be more until September unless they come in with a small amount of "playoff stock".
Also the reason I've heard from a lot of people why the white jerseys don't sell is because there are 30 of them, every team has a mostly white jersey. Sure other teams use red, but it at least sets jerseys apart a little bit.
You can customize tees and sweaters on nhls official store. One of the designs you can pick is the O, and you can pick where and how big.
I get so frustrated trying to find Sens gear. Ive been to Ottawa once and found awesome gear, but online its pretty slim pickings.
Ooh. Now that's a good idea that I hadn't considered... I'll look into the NHL Shop option. I want a Pageau O t-shirt, as I'm not yet convinced that he's going to stick with 44 (i.e. I'm not in a mad rush to get my heritage jersey customised).
But yeah, I live in Ottawa and I find it challenging to find Sens stuff that I like. There are so many things that are covered in all sorts of other designs, and then have a logo in the middle of it all.