Sorry, previous post was corrupted, here is a second effort.
A stretched my frame a few seasons ago and here is what I was able to find then....
The aluminum tube used on most frames is designated schedule 40 6061-T6. Depending on who made the frame the they used one of the following sizes. Measure carefully and be sure of what you have. (note: nothing on a 1 1/2" pipe will measure 1 1/2"!)
1 1/2" nominal (1.900", 48mm o.d.)
1 1/4" nominal (1.660", 42mm o.d.)
SpeedRail fittings are made by 'Hollaender' here is a link to their pipe sizing guide. http://www.nurail.com/2010/images/nav-on_09.png
ALS Industrial Supply, 303-355-2391 was the best price I found on fittings in the Denver metro area. You will need to go on the Hollaender web site and pick out the fittings so you can order them by item number.
DRE in Wheatridge will sell tubes of either diameter cut to length... http://www.downriverequip.com/asp/product.asp?product=807
I also found a metal distributor in Brighton that was willing to sell cut lengths to the public. They normaly deal with professionals so do your homework, they have little time for 'noobs'.
ALRECO, 11299 Brighton Blvd, 303-287-7210
Note: the tubes sold by DRE are anodized, the ones from ALRECO are not. The non-anodized tubes will rub off on stuff leaving a dark grey mark that can be difficult to remove from your raft.
If you only need to stretch things a few inches you might consider a 'break down kit" type of splice...
This is a solid bar that fits inside the tube and is pinned with through bolts. You could get (or fabricate) an oversized one that would easily add a few inches.