Mine came free with my veil (ordered from Bianca veils, etsy).
You could also request your seamstress to make one from the leftover fabric from your dress alterations. Most likely that will either be free or something reasonable, and you know for sure th fabric is good quality.
I know a lot of people get them on etsy. I know my mother sells bows to a woman who makes them in an etsy shop and the woman apparently sells many but I'm not sure of what the name of the shop is. I would just shop around there.
I got a toss and throw garter from Etsy for $25. The seller was Bridal Blossoms Shoppe in Nashville. I'm plus size, so I wanted to make sure it would fit. The one I got has a rubber-like thing on it to keep it from sliding down so it's not going anywhere, I think. It was measured by the inch of your thigh. Granted, being plus size, I have meaty thighs and a thin garter doesn't look super-duper sexy on me, which was my own fault for not thinking that one through, but I didn't want to spend $50 for a plus size one elsewhere. Besides, I like them SO much anyway and no one else will see them but me! Plus, you could customize colors and styles. Etsy was a total win for me!
I also saw a BUNCH in Hobby Lobby when we purchased our cards box and vow books. They were maybe $10-15 and very traditional-looking (white or pastel blue with lace). I'm not sure if they were no-slip, but if they aren't, I highly recommend putting it on the garter anyway.
The one I got from Etsy is a navy strap with a silver chain and white bow, because I didn't want lace since my dress is more "bling" styled. I also don't like pastel blues, so I wanted to go with a more custom color.
I'm wearing my moms garter as my something old and I also got a free plain white garter that came with my veil to wear as a toss garter so I more than likely will not be buying others but I loved the garters on Etsy and that's where I was going to buy them before my mom found hers.
Dollar tree...for a dollar. They have white, ivory and even baby blue in most stores. We decided not to even do one because it looks like rolled panties when coming out from under the dress...lol...but that's my opinion and also based on my experience when I married my late hubby 15 years ago.