You can fetch them directly from Uniprot, if you know the uniprot ID the fasta sequence can be retrieved from the URL https://www.uniprot.org/uniprot/{UNIPROT_ID}.fasta

So, if you have a file with the IDs (one per line):

for ID in $(cat file_with_ids.txt); do wget https://www.uniprot.org/uniprot/$ID.fasta; done

You can upload your list of identifiers to the UniProt batch retrieval tool at https://www.uniprot.org/uploadlists Please don't hesitate to contact the UniProt helpdesk if you have any additional questions.

import urllib.request 
my_uniprot_IDs=set(open('my_uniprot_IDs').read().split())
template='https://www.uniprot.org/uniprot/%s.fasta'

outfile_fasta    = open('my_uniprot_IDs.fasta', 'w')
outfile_obsolete = open('obsolete', 'w')
outfile_success  = open('success', 'w')

for anID in my_uniprot_IDs:
                                     try:
                                        this_url=template%anID
                                        fasta_text=urllib.request.urlopen(this_url).read().decode('utf-8')
                                        outfile_fasta.write(fasta_text)
                                        outfile_success.write(anID+'\n')
                                  except :
                                        urllib.error.HTTPError:
                                        outfile_obsolete.write(anID+'\n')

   outfile_fasta.close()
   outfile_obsolete.close()
   outfile_success.close()