Reminders in notification center/lock screen without time/day set
I don't think there is a way to get reminders to appear on the lock screen without putting either a time or a location stamp on them.

However, if you know where you will be doing this shopping, then the location-based reminders will actually work well for you. Just set up a new contact with the store's name and address (and, optionally, the phone number if there's any chance you might ever need to call them). Then you can use the "Remind me at a location" option and select the name of that store. Make sure you select "Arriving at" rather than "When I leave". Your reminder will go off the next time you go to that store regardless of what day it is.

Unfortunately this will only work if you do know where you will be doing this shopping. If you were thinking of something like "Remind me to check the oil next time I gas the car", then I'm not sure what to suggest aside from doing a "when I leave home" reminder (which would have to be refreshed every time you leave home for a different purpose and which therefore would probably be more trouble than it's worth.) 

I've used this technique several times, simply by entering the names and addresses of my local shopping mall and local grocery store into Contacts, then hitting a location-based reminder. This is actually how I do my grocery lists: whenever I need to add something I just edit the reminder. Next time I go to the grocery store, there's my list right on my lock screen. If it's a long list I might need to unlock the phone to see the whole thing, but overall I've found it works great. (Certainly much easier than working with a handwritten list which I had a bad habit of forgetting to take with me when I went shopping....)

** Please note, though, that the size of the geofence depends on many factors and your phone might consider you to have "arrived at" a particular location when you are actually a few blocks away. This has occasionally caused my grocery list to pop up prematurely if I'm just passing by the store rather than going in.

Hope this helps!