So I know that the thread is kind of dead by now, but I thought I would add my 2 cents for anyone that comes here after me.
You can certainly enlarge your urethra enough for a finger via means of sounding only, although, this takes an extremely, extremely, long amount of time. You usually want to get a sounding set, don't get the guyon ones or any of them that are extremely curved. The Hegar ones are preferred for stretching imo, they have a thicker and a slimmer end on the same sound, allowing you to put in the slimmer side and work up to the thicker end.
Before moving to the next sound size you should be able to fit the thicker end of the previous sound without applying any force, be it putting the thicker end directly or moving from slim > thick. Be advised that working up to bigger sounds does require a little bit of strenght to be put in, but not enough to just jam it there against its capability. Do this according to your own pain threshold.
Lube is extremely important, there are special sterilized lubes (some even containing a numbing agent) that can improve the experience greatly. Just be careful of not hurting yourself if a numbing agent is used.
I would suggest allowing a minimum of 3 days between sound stretch sessions, with a week being ideal. You do need to allow some time for your body to recover.
This might be a personal case, but I've been putting stuff in there for about 7 years now. I am really close at fitting my index finger inside my urethra now.
If you don't want to wait long, a meatotomy would be your only other choice. By all means. this is a surgical procedure and should be treated as such. If you appreciate your own penis you will sterilize everything and get specialed equipment such as surgical scissors and a hemostat. Also get some numbing cream for good measure and keep the area clean for a at least a week after the procedure to avoid infections.
Specific info on this procedure can be found on the link below.
WARNING: the link contains graphical images that might not be suitable for everyone.
In any case, have fun and be safe.