FWIW, I just went through replacing a 100 month Kirkland battery from Costco and you’ll not beat the price of a replacement. I got 94 months out of the battery so taking back the old one the next Costco trip got me a 10% refund on the Kirkland battery plus the $15 core fee paid when purchasing the replacement back. The warranty terms have changed on the newer interstates but it’s still a bit over 3 years.
The sizes are a bit more limited at Costco, but you can typically find something “close enough” to work. This page has much of the batteries that are available at Costco and how the Costco group number relates to the BCI group number. The batteries with a posted price were in stock at 4th ave S a few weeks ago. I got a 40R battery for $73 plus tax.
A full rundown of sizes and post positions available from Interstate is here.
A little bit of google and looking at what several sites recommend for a battery should tell you what your car originally came with for a car battery.
