I buy pretty much all my bottles from ebay. For DIY mixing the soft LDPE dripper bottles work best to decant all your flavour concentrates into. The cheapest bottles normally come from China so you might want to pay attention to the estimated delivery times.
Another thing to pay attention to is how wide the neck opening is because when you're decanting liquids between bottles the ones with narrow necks can be a nightmare to work with. Gorilla bottles have wide necks and thin drip tips which makes them easy to decant your finished recipes into while still having a drip tip that will fit the fill ports of most attys.