Welcome!
I'd grab some wheels from the first generation Honda Insight. You can probably get a set for under $200, make sure you get the center caps included because people want too much money for a set on eBay. They are light weight, aerodynamic and look the part of an ecomodder.
After doing some searching they appear to be the same size as a 14" wheel that comes on the Spark. You'll want to verify that. The tires will be much smaller if they are included but are possibly the lowest rolling resistance you can buy. It would throw off the odometer but you may be able to use them as well.
Pretty cheap at a u-pull-it style junkyard if you can find them there. Otherwise try
https://www.car-part.com/
Some people get even more extreme and use the spares from that car on their Insight because they are even lighter and smaller.
The only issue in going with a 14" or 15" wheel now is the seemingly limited choices for tires that have a low rolling resistance.