First off, your fear of walmart clerks. you don't have to be a certain age to buy lubes or condoms or vibrators. All you need is money. I started when I was about 15-16 (I'm 18 now) I was the same way, nervous about buying that sort of thing but you've just got to keep thinking "Hey I'm having sex tonight!" That always helped me. and then the nervousness will go away and it'll feel no different than buying milk.
now onto the types of lube, you probably don't want to use anything with a warming agent in it because warm makes you bigger. i personally would recommend Astrogilde it's in a purple bottle, it's not quite as slimy as a jelly, but it keeps it's lubricating properties best out of everything I've tried. You may try a mentholated lube this causes extreme stimulation for the girl and cooling for you, this is not for every girl however, it can irritate some girl's down stairs. So if you decide to go that route apply conservatively. after you get pat this you'll be knocking down sexual barriers with ease. Just takes a little trail and error. hope I was a little help .