I was wondering if using fruits from a supermarket would make a good wine or not. At the moment I have been buying frozen berry packs which work great but I am looking to branch out into more interesting flavours. If not, where would the best place to get fresh fruit be?
The problem with much fresh "Supermarket fruit" is that it is usually picked early and is usually unripe. If one can find "ripe" or "over ripe" fruit in a supermarket then IMHO that is the fruit one really wants to use. Unfortunately such fruit has limited shelf life and so is usually difficult to find unless it has been marked down for quick sale. The most dependable supermarket source is often (as noted) frozen fruits but not every fruit freezes well, so the choice can be limited.
One can use unripe fresh fruit but the neither the sugar content or the flavour is fully developed and so it can be a wasted effort to attempt to produce wine from such materials, that or it produces an inferior product.
If one can form a working relationship with a fruit seller (eg shop or stall at a "farmers market") then it is usually possible to ask them to keep back any very ripe fruits or to buy such things when they see them at the distributor or find them on the farm.
If one is enterprising enough then an early morning trip to the local fruit wholesaler or wholesale market can yield some great finds of ripe fruit at even better prices.
The main point being one needs ripe or over ripe fruit to ferment and that fruit tends to have the lowest cost. Prime but unripe fruits have a higher cost and don't ferment as well. Out of season fruits should only be used if one can find a bargain price or it is absolutely necessary to use that fruit.
Yes, you can make good "wine" with supermarket fruits, but as you mentioned they are not always the most fresh resource available.
Local fairs are a good source of fresh fruits, etc ... Not sure if your city has regular fairs, but in these places you can find lot of good stuff and also organic fruits.
When I make melomel, flavored Kombucha or cider, I always buy the fruits at fairs. This is also a way to support local producers.