At the DCP page you read about sending your DCP fysically on a disk. Ever more festivals and cinemas accept internet based transfers. Some festivals even deploy their own dedicated upload service. If a festival accepts internet based delivery but does not offer an upload service, you may consider using one of the available transfer options. Don't expect this to be a cheap alternative for fysical delivery, some may be faster though.
There are countless services providing file transfers. I did not find a reliable free service capable of handling file sizes above 5 GB. Typical DCP sizes are much larger, so you will need a paid subscription if you choose to transfer your DCP. If you have a paid file sharing account you may try to use that at your own risk. You better first test if files arrive unharmed. When choosing from available transfer services best results i get and what people report are with Filemail and Aspera. Filemail is reported as slow but reliable. Aspera is a much faster service (by IBM) and needs dedicated software at both the sender and the receivers sides. The only "free" option you might consider is setting up a ftp server yourself. First of all check at the festival or cinema what they accept.
Filemail has a pro option at a fee of 15 euro a month (jan 2021). The free option only works for file sizes up to 5GB. The 7 day free trial option might help you if you need a transfer only once.
Aspera comes at a price of over 700 dollar a year . It also has a 1 US dollar per GB option for a maximum of 10 users. Aspera has a 30 day trial option. Be aware that it will be necessary to send clear instructions to receivers about how to get your DCP using Aspera.