I bought a fountain bag-in-box off of ebay for like $25.

Something like this: https://www.ebay.com/itm/DR-PEPPER-2-5-Gallon-Bag-box-Soda-Syrup-Brand-New-Box-SEALED-MSRP-42/324225299176?hash=item4b7d5572e8:g:wscAAOSwZcRfvkJM

These bags have a special tap specifically designed for the fountain that they will be connected to. There are different types of connectors. People have designed taps that are basically the bag connector on one end and a simple tap on the other. I ended up buying two different taps off Amazon and one worked.

This is the one that worked for me :

Dr. Pepper BIB Tap Valve, Small, Red https://smile.amazon.com/dp/B01GBMPXHU/ref=cm_sw_r_em_apa_fabc_KP5VE5VFMWR96HJZCTGA

The syrup is thinner than other syrups I've used, and the bubbles aren't quite as strong as I'd like with this syrup no matter how much I carbonate. But it works, and I like it well enough.