Some options for the pre 1999 model year and prior group:
Husaberg used to sell a large 5 gal or so behemoth tank. They appear on e-bay now and then.
Husaberg also used to sell a number plate tank. They may be still be available new from some dealers ( spotted some on an Austrailian web site).
If yours is a non-estart model Eric's of Pasadena used to sell an aluminum under the seat tank, again check e-bay.
If yours is an FC you could also use the Fe tank which is a bit larger than the new for 97 small MX tank.