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/

Amazon! [This ](Brhttps://www.amazon.com/dp/B01DN6001G?ref=ppx_pop_mob_ap_share) is the link for the bottles, I also bought metal funnels from amazon (pack of three). The bottles are dishwasher safe (the glass bottle part, not the lid) which makes them convenient.

Brieftons Glass Water Bottles: 6 Pack, 18 Oz, Stainless Steel Leak Proof Lid, Premium Soda Lime, Best As Reusable Drinking Bottle, Sauce Jar, Juice Beverage Container, Kefir Kit - With Cleaning Brush https://www.amazon.com/dp/B01DN6001G/ref=cm_sw_r_cp_api_i_2gsiEbTDPWY0N