You can pre-book the tuxedo on-line, it's under gifts and services on their website. A tuxedo and 4 shirts is $120.00. I couldn't find a faq about once you're on board. You might be able to call them. Hope this helps!
For the price of that rental, you could actually buy one and have it for future use. I saw brand new pants and jacket at JCPenny for $99, then the shirt and accesories (tie, cumberbund, cufflinks, etc) were for about $39....that's $138 + tax would be around $150. Just think of all the future cruises you could wear it to! Good luck.
Better Yet, If you want to buy your own tuxedo, you may want to check out the thrift shops like GoodWill or the Salvation Army. Sometimes Tuxedo Rental Companies get rid of their older stock, and donate them to charities.
I have lucked out several times
I was able to get a 1 button, notched lapel Tuxedo at the Salvation Army for $40.00 CDN
(I recognized the Company Name, we haven't had it in Canada for 15+ years but it is the same
style as what Carnival rents on the ship)
Another time I went to the Mennonite Central Committe Thrift Shop, and they had several Tuxedos available, I bought a short waist coat (similar in style to a military mess jacket) and pants for
$10.00 CDN. It cost me more to dry clean it than to buy it