Where can one convert their bitcoins to Liquid LBTC bitcoins on Blockstreams Liquid Network? Is there a place where one can convert their Bitcoin to Liquid BTC? Which exchanges? Is there even a mobile wallet that supports it if one wants to withdraw it?
According to the documentation, anyone running a Liquid node can generate and fund a peg-in transaction. And anyone can run a Liquid node.

You'll need a participant, which can be an exchange, to peg-out and exchange your L-BTC for BTC though.

The peg-in is explained in more details here.