You can also buy your domain for about $9 (a year), then have it point to tumblr site. This way, you get free web hosting via tumblr now and you still have the option to get a dedicated web host later on.
Alternatively, you could get a domain for your own name, and use a subdomain to your webcomics. For example, if your name is Jack Sparrow and you own "jacksparrow.com", you can create the subdomain "mywebcomic.jacksparrow.com" to point to your tumblr account. This way, you only pay for one domain a year, and you can use many subdomains for your projects.