And look at this guess who's back
If you're asking how to get specific items, then this is how.
If you haven't entered a shop yet, you can save before you go into it. If the shop doesn't have an item you want/need, just exit to the menu and click continue. You should be able to go into the shop and see different items. If the shop keeps showing the same items, move to any empty spot, save, and enter the shop. This is how I get stuff for some of my builds. You can do the same with secret rooms, but it won't change the event.