20oz is an in-between size. You might have an easier time finding 16oz, 24oz or half-liter. Some options (I haven't purchased these; I used 16oz "French glass' bottles with squared corners and stainless steel lids but mine weren't air-tight; I just kept 'em upright in transit in an insulated lunch carrier):

1. People have already mentioned mason jars (usually in 16oz or 24oz sizes). You can get leak-proof lids for them, but would probably buy that separately-be careful to match the size, either wide-mouth style or standard style. If you want 20oz exactly, something like this is similar but personally I wouldn't want the handle taking up space: amazon.com/Tazas-de-tarro-Mason-Multicolor/dp/B0752X8DQ6
2. 18oz bottles with stainless steel leak-proof lid: amazon.com/Brieftons-Glass-Water-Bottles-Stainless/dp/B01DN6001G
3. swing-top style bottles (this one is 16oz): amazon.com/SXUDA-BPA-Free-Airtight-Stoppers-Beverages/dp/B083HZYJJ9/