The place to file Form 48 depends on the specific form you mean, as there are different Form 48s for different agencies. Here are the two main contexts from the search results:

• Form 4868 (IRS Application for Automatic Extension of Time to File U.S. Individual Income Tax Return):
  You file Form 4868 with the Internal Revenue Service (IRS). The mailing address depends on your state of residence and whether you are making a payment with the form. For example, if you live in Alabama, Florida, Georgia, Louisiana, Mississippi, North Carolina, South Carolina, Tennessee, or Texas and are not making a payment, you send it to:
  Department of the Treasury, Internal Revenue Service Center, P.O. Box 1302, Charlotte, NC 28201-1302.
  If you are making a payment, send it to:
  Internal Revenue Service, Austin, TX 73301-0045.
  Addresses vary by state; you can find the full list on the IRS instructions for Form 4868

• Form 48 (Regional Income Tax Agency Business Registration Form):
  This form is for business registration with the Regional Income Tax Agency (RITA) in Ohio. You complete and sign the form and mail it to:
  RITA, ATTN: BUSINESS REGISTRATION, P.O. BOX 477900, BROADVIEW HEIGHTS, OH 44147-7900.
  You can also register electronically through the RITA website

If you meant a different Form 48 (such as related to customs or other agencies), please specify for more precise filing instructions.