I'd go with 6x Maxwell BCAP0350 (350 farad). It's just big enough to start a car on its own. Any bigger probably isn't necessary.
This site supposedly has them for $14 each plus shipping, but you have to ask for a quote:
BCAP0350E270T11 Maxwell Technologies Ultracapacitor- Richardson RFPD
This site also has 'em for $14:
https://www.verical.com/quote-cart
Mouser has them for $17 each:
https://www.mouser.com/ProductDetail...20Technologies
They come in 2 flavors: solder post, or weld, meaning you have to spot weld the connections.
Ebay might be the best place to shop:
Balance board (although balancing is not necessary)
https://www.ebay.com/itm/Six-In-Series-MaxWell-350F-2-7V-T11-T10-Supercapacitor-Balance-Protection-Board/263928360582?epid=1950825530&hash=item3d735b2e86:g :g-sAAOSwqrZa1pLZ:rk:25
f:0
EDIT: Just found these 400 farad Eaton for $12:
https://www.mouser.com/ProductDetail/Bussmann-Eaton/XV3560-2R7407-R?qs=sGAEpiMZZMuDCPMZUZ%252bYl72C5N2%252bSzT2g2g0v E9ei8Y%3d
Also found this 400 farad for $12 and it has radial leads to solder to. New item it seems:
https://www.mouser.com/ProductDetail...yX1%2fDaf4o%3d
I use
octopart.com to keep an eye on electronic prices at various retailers.