Myself, I don't mind attending a shower for a second child, especially when they are spaced far apart, but the fact is that if you have a second shower *someone*, at least one person, will be grumbling behind your back! And I'm no "Miss Manners", but really it's not *exactly* proper to throw a shower for yourself, because it can create the appearance of inviting friends over for the purpose of giving you gifts.
You might follow the previous parent's advice and look on Freecycle or Craigslist for free or low cost baby items. Just think- you probably gave your baby things away for nothing or almost nothing, and other people are doing the same thing. Many items will be barely used or at least fantastic bargains- just yesterday I saw a Pottery Barn crib listed on Craigslist for $20. That's pretty unusual-and you're short on time!- but if you look around you can find good things that are relatively inexpensive. Check your local resale shops. You may even have friends or relatives you can borrow things from.
That being said, it's nice to celebrate the arrival of a new baby. Maybe you could host a little party at home or at a favorite restaurant, and ask guest to write a welcome to the new baby in a baby book. And if someone asks you what you need you can thank them for asking and list a couple modest items. Your family will surely be generous. Plus, as a second time mother, you probably noticed the first time around that most of the things you think you are going to need you don't end up needing. A crib, some jammmies, and a car seat are just about all you need in the beginning.
I'll bet that if you're honest with your Mother In Law that she will offer to throw you a shower. I don't mean to be critical, but if you are concerned about doing something that appears to be not exactly right, you're answering your own question to a certain extent. That's what I'm getting from your post.
I really do hope that you get the things you need one way or another! Good luck and congratulations to you on your baby!