How do we get an 1098 form. another lender took over our mortgage since last year

You must know who you are making your mortgage payments to --- did the new lender not send you a 1098?   Or did you not receive one from the old lender?  Have you contacted the lender?

Entering the 1098 info is only used if you are itemizing deductions ... so if you use the standard deduction you can skip entering in the mortgage interest. 

And most mortgage companies have that info either online or thru the automated system.