You can't buy direct from Mitel, find a dealer to handle for you. If you give dealer the ID of existing card he can theoretically transfer the existing license to a new card, then ship it to you. There's some paperwork involved, he'll have to take over your account from whoever had it before. Card itself is a couple hundred dollars.
Another alternative is to buy a used 5000 off of E-bay, I see them for as little as $70, and take the card from that. But beware, all the license info is on that card. So be sure you get one for the proper chassis (1U CS-style or 2u HX-style) and enough phone licenses of the proper type to serve your needs: 86XX phones need class A license, 53XX need class D and digital need class E. If you tell us what you have (chassis, phone types, trunk type). If you're not sure find pix on Google Images to compare.