The only bulbs that put out UVB are called mercury vapor bulbs. ZooMed Powersun and T-rex Active UV are the two most popular brand names. They are very expensive compared to the tubes, but last much longer.
The gases inside the tubes that produce UVB are used up long before the tubes stop giving off visible light and so the tubes need to be changed after 6 months at the very most. The mercury vapor bulbs are good for at least a year and some reports say two years.
With the bulb you have now, your iguana is at risk of not being able to process calcium correctly and developing metabolic bone disease (MBD). You need to get a good quality UVB light very soon so your ig can digest food properly and have healthy bones and muscles. Here's a well-written page where you can read more about MBD. The Green Iguana Society website in general is an excellent read.