I use any charms I like. Swarovski, Coach and Micheal Kors have nice charms. I also saw some cute charms today at H&M. They only reason I didn't pick one up was because I was afraid it may cheapen the look of my LV. I also found a pretty vintage large gold pendant that I attached a chain to that I use. There are many less expensive options for bag charms. My favourite so far are LV Bandeau's. They are about $140 but beautiful silk and made in Italy. I love the way they look tied on the bags.
I started a thread awhile back about how we decorate our LV's. If you click on my name, you can see the threads I've started and you should be able to find it. I'm not sure how to link it up to this post. HTH
For a pop of color, I typically use a bandeau but have been using my multicolore pastilles charm lately. I love all those pastel colors. I also have a bright orange Graffiti charm that I use once in a while --I've seen a few of those lately--pre-loved on the bay/bonz---for about $225-250 though.
For less than $100--you could do Juicy or Coach. Or even get an LV Luggage tag with your initials--you could do the initials with a bright color like blue. They're about $52 with tax.