#For wearable projects 18650 lithium cells tend to be too large. I see tons of project using flat lithium packs instead or just non-rechargeable alkaline batteries. I don't like flat pack lithium batteries since they are easy to puncture and with a hard case they are almost as large as 18650s.
There is a good alternative which is a good compromise between size, sturdiness and capacity: The cylindrical 14500 lithium battery. 14500 is essentially the same size as an AA alkaline battery. From what I can gather there is pretty much only one company which sells them in quantities in the US: Tenergy. These batteries have a 800mAh capacity which is pretty good for their size and they are fairly sturdy while being small enough. I buy them in batches of 100, at $3.69 a pop. Also, when you order 100 you get them in their original neat packaging. Product page: http://www.tenergy.com/30001 Tenergy has an online store where you can buy direct: https://power.tenergy.com/tenergy-li-ion-14500-cylindrical-3-7v-800mah-flat-top-rechargeable-battery-ul-listed/ They are based in Fremont, CA. Reputable company from what I can gather, never had an issue with them.
Highly recommended for portable projects where 18650s are just too large.