Larger comic shops like Midtown Comics and Things From Another World offer online stores that sell recent comics and variants. They also offer subscription services so you can get them mailed to you, often at a discount.
If you’d prefer to buy comics from local stores though, you can ask the people at the LCS to order Black Widow for you. Most of the time they are happy to do this, and it means the store can tell Marvel that there’s interest in the title.
For anyone looking to buy comics and learn about the options out there, my friend wrote a great Comics 101 guide.