Optima makes
round-core nickle flats. Your profile doesn't say where you are -- if you're in the US, I'll add that I couldn't find a US retailer with them in stock so you might want to directly contact a strings specialist that already sells Optima products to ask if they can special order it. (I suspect the answer will be "hopefully eventually".) If you're not in the US, access should be easier.
Dogal makes stranded-core nickle flats, but if the TI Jazz Flats are difficult to afford, forget I mentioned these
The TI Jazz strings have a lot of loyal fans here. Do a quick search and you'll find plenty of threads about them. My take is that the initial purchase price is high but they'll last considerably longer than most strings, which can make them cost-effective over the long term.