I had mine done about two months ago at RS at the ACC and they said they had a good supply of numbers and letters at the time but they were slowly getting used up --- I doubt there was a huge run on Onyx jerseys being crested all of a sudden so you should be ok --- I don't think they are getting any more of the letters/numbers in stock, so I would suggest the sooner you want to get it done the better --- obviously numbers like 1, 0, and 4 tended to go quicker than others and letters that happen to spell the name GIOVINCO are probably being used the most too!
No idea about BMO as I didn't check last week....If I go to the game tomorrow I can ask if you aren't heading down to BMO.....