There are people here on the forum who sell them for quite reasonable prices, I think I bought mine for about 60 TBT from a kind user named Mayor Kaleigh. (My current signature) But there's many other users who do signatures apart from her! Like Cass123 listed above there's also pokemanz who does signatures too. Although, it depends if they have slots open for you to order and such, sometimes they may not have slots open at the moment so you have to be patient. Also, price varies from user to user, so some prices may be different. Good luck on getting a signature!
I got mine from pokemanz, he runs a shop on the Tbt section.
Really nice guy and you can pretty much choose whatever you want and he will work it out for you
There are a lot of options though, I'm just biased because I love mine haha