Can I buy Comodo positive ssl for webadmin login?

You can get a CSR from the firewall. If the CA offers a signing process for CSR, this should work. 

Thanks for your reply. Yes, I got the comodo ones, but for some reason, it cannot be selected in the drop down box for the admin settings pages. Any ideas?

Thanks for your reply. Yes, I got the comodo ones, but for some reason, it cannot be selected in the drop down box for the admin settings pages. Any ideas?

You likely did not upload the CSR properly. Did you use "Import" on the right side of the CSR or did you upload the PEM to the firewall? You need to use Import. 

yes, you got to use the import button in the CSR line to complete the upload.