I am also a happy owner of the Yamaha GL-1 (with a self-installed Mi-Si pickup). At the time when I bought mine (Dec 2012), there were no others I knew of for sale. Now Cordoba has one selling for only $10 more ($109) at Musicians Friend.
I've had D'Addario EJ27N (normal tension) classical strings on mine. These are marketed as 'student grade' but the sound is great and they are effortless to play on. They last quite a while and when they are due for a change you'll know because typically the G string sounds dead and 'thud-like'.
Be advised that D'Addario also has a set EJ27H (hard tension) which I would caution you against using in a guitalele, which is pretty much a 1/4 sized guitar.
Keep in mind that by using regular classical guitar strings, you are taking strings designed for EADGBE tuning designed for the full size scale of as classical guitar, and tuning them UP a 5th to ADGCEA tuning, and as such, if you use NORMAL tension strings designed for the full size scale of as classical guitar and tune them UP for the guitalele, you will in fact ALREADY have much higher tension.
On my full size classical, I use the harder tension EJ27H strings, and going to the Yamaha with the normal tension EJ27N strings the feeling is almost the same. Either set cost like $5 online and/or retail and can be had pretty much everywhere.
At some point I plan to try the Thomasik-Infeld chrome flatwound set called CF128, which has 4 wound strings and 2 plain nylon strings, and by being flatwound, has almost no string squeak/finger noise according to fellow UU brother OldePhart (John).
The string squeak drives me nuts.
Also, I've seen that Aquila has a set designed specifically for Guilele/Guitalele, and they are available at most places that have the Aquila uke strings, also like $5-$7 per set...but I have not tried them yet.
Just for kicks, I replaced the 4 higher-pitched strings with the Martin M620 tenor uke strings, with the high-G, and it sounds very open and happy compared to the linear standard with what would be a low-g. Not sure if I will keep this setup, but next string change I am going to try the aforementioned Thomastik-Infeld CF128 set. They also have a set called CF127 which has 3 flatwound and 3 nylon strings like most standard classical sets.
-Booli