Making underpants isn't that bad. I did it back in the day in high school (1975) . Still have my little box of elastics and pretty little embellishments. All in perfect condition! It wouldn't be that easy to find cute fun ideas today though. I looked online to see what was out there. Check sewsassy.com They sell all the stuff you are looking for.
From what I recall, tricot fabric is very wide so you could possibily get quite a few pair for your money. Who sews to save money anymore anyway?? Isn't it for the fun of it??!!