Most mobile networks provide a mobile IP. If you want a fixed IP you should still check with your provider first and see what they can offer; it's also a good idea to type a phrase like 'fixed IP mobile broadband' in a search engine and see what advice you can get - this will link you to user forums and comparison sites that talk about the best deal.
One thing you may be able to do is get a fixed IP SIM card - you can see some details here. You would have to pay for this, though tarifs vary according to how much internet you use.
You should also check whether a fixed IP is the best option for you, whether you need one and what the advantages and disadvantages are.

Fixed IP which is also known as Static (or) Dedicated IP. To get Fixed IP, you need to contact the Network service provider. Normally Static IP never gets changed of its IP address whenever you reset (or) unplug your modem. Static IP can be changed only by requesting the Service provider. To check your IP address is Static(Fixed), visit the site IP-details.com .