Is there an online store or website that you can create a cassette mix tape?

[livewyre]

I think it's pretty safe to say this is not going to be a service you will find anywhere on the web - plenty of sites to do the switch the other way around, but I have never seen anyone offering the service you are thinking of.
One issue here is that the compressed MP3 version of the song would be lower 'quality' than would normally be recorded onto analogue cassette - you would normally record from a CD or maybe from vinyl - MP3's are a great portable format, but there is a 'price' to pay for the portability, and that is that the files are 'skinny' there are elements missing that are removed when they are compressed.
More about MP3 'quality' issues here:
http://www.convertmycassettes.co.uk/mp3.htm