There are two options if you're looking for "alternatives to designer clothes" in my opinion.
Either you opt for outfits that are in not created by famous design houses. That doesn't mean the clothes have to look bad though. In many ways, designer clothes are more about the brand and image behind the clothing, rather than the garment itself.
Your other option would be to seek out old season's designer clothing, second-hand designer clothes or wholesaler clearance sales, which tend to be great places to find designer clothes for "less than designer prices".
If the second option sounds appealing, then the internet is your friend. Their are a plethora of apps and websites dedicated to helping you find good deals on your favourite items.
And stores that sell designer clothes also often have sales and clearances which they will advertise on their site/app too.
Finally, I would also advise you that much of the clothing lines you find on the "high-street" are often based on designer trends anyway. So even if you can't afford the original designer outfits, their style and themes will most likely be in a store near you for a margin of the price anyway!