This app was mentioned in 3 comments, with an average of 3.67 upvotes
Where do you live?
You could go to one of the big Hardware Stores ("Baumarkt" like OBI, Bauhaus, Hornbach,..) but they are normally not in the center.
You can also just search in Google maps for "Eisenwaren" or "Werkzeug" to find a smaller shop closer to your home.
Also there is a good app for finding any kind of shop in vienna called "Öffnungszeiten"
Edit: link
I can recommend this app to find (open) supermarkets in austria:
android: https://play.google.com/store/apps/details?id=at.appscape.oz&hl=de
ios: https://itunes.apple.com/at/app/%C3%B6ffnungszeiten-%C3%B6sterreich/id399594904?mt=8
Here are tips I can think of right now, some more general than others.
Shop at Hofer, Lidl and Penny. Avoid Spar, Billa and especially Merkur. The selection at discounters is smaller though, so if you want something special and exotic, be prepared to take a second trip to one of the more expensive stores after checking if Hofer has it.
"Special" shopping opportunities like farmer's markets didn't beat the prices of discounters whenever I checked. Maybe there's markets I don't know about, but most seem to cater more to the hipster crowd than to people doing actual day-to-day shopping. I'm happy to be educated by ppl who know more though. :).
Get the app Öffnungszeiten Österreich (Android/iPhone) to see where certain stores are (and which are closest to you), and when they're open.
Another thing about opening hours, when you're shopping at discounters, try to get your bread, vegetables etc. in the morning, since in the evening, high turnover items are sold out sometimes. Same on saturdays. Also, try the fresh dark breads they have in the Backshop corner at Hofer, they're relatively cheap and taste great! Oh, and get a shopping cart and be prepared to throw everything back into it at lightning speed after scanning at Hofer cash registers. Kinda stressful, but that's the price we have to pay for the, well, prices.
Get the app Aktionsfinder (Android/iPhone) if you want to compare prices and check for ongoing sales.
Eat lots of pasta and potato dishes; also dark bread with butter or bread spread. Or get instant ramen at cheap asian stores (oh so healthy!). No really, check out Nakwon or Prosi if you ever have the time and like to cook Asian stuff, some things are way cheaper there compared to anywhere else.
Join /r/eatcheapandhealthy - try one of the burrito recipes, I recently made 60 in advance and froze them, cost me about 35 euros and lasted for 30 meals.
When out and about, eat at Deewan, Gagarin, Rice & Noodles (close to AAKH Campus), or try the cafeteria (Mensa) at TU or Afro-Asiatisches Institut. Or try to survive on one Kebap a day, worked well for me for a time, heh. :)
Enjoy your time in Vienna! :)
edit: fixed typos, added links