Well, no real way go get cheap lithium batteries. Even salvaging them from old power tool batteries cost money. As a quick calculation, you would need about 100ah at 36v to get what you get from your 105's. For 18650 cells that would be something like 10s 50p depending on the quality of the cells. That is in the 500 cell or more category. You will learn all about battery tab welding if you go that route.
You might try to get pouch cells from a retired Nissan Leaf battery or something equivalent. They have more capacity then small 18650's.
Another option is RC Lipo, which is the most cost effective of all new lipo. However, mishandled it can burn your house down. So learn all about RC lipo before you go thought route.
Favorite Quote: "This is L.A., sugar. There is no 'over the top." --- Chris Erskine
Current build: Liahona w/ cheap front suspension and suspension seat post. Yescomusa 36v 800w generic front hub motor. 15ah Headway triangle mounted pack. Tronsung 30 amp,
Previous Build:1992 Trek Antelope 800 - Bone Crusher (no suspension) - Yescomusa 800 watt 36 volt front wheel kit. Don't do it! Get suspension!!!