Can't speak for the tension on the Sonores, but if you want the historical sort of string gauge, the modern medium gut G is closer to what one meant to be used as a Solo A was like.
Otherwise, I know Gamut sells a set specifically for solo tuning, and people have said good things about them before. Shouldn't be too difficult to get just the highest string I'd imagine.
Just to follow up on this, linking back to
a post I've made before and giving some more information:
That post has links to other posts on the site talking about this same sort of thing, and from the first one I linked in there, I managed to find a
scan online of Milushkin's bass method and I copied down the gauges that were listed there, in millimeters, for orchestral and solo tuning, with gut strings, with some of my own little notes and conjectures added on:
Orchestral:
G: 2.30mm—2.50mm
D: 3.00—3.20
A, Overwound: 2.90—3.10 (core 2.30—2.50)
A, Plain: 3.50—4.00
E (overwound only): 3.90—4.10 (core 3.00—3.20)
B/C: 6.00—6.50 (core 4.00—4.50)
Solo (probably at solo tuning, roughly equivalent to "modern" gut string gauges):
G: 1.90—2.00
D: 2.30—2.50 (same as orchestral G)
A, Overwound: 2.30—2.50 (core 1.90—2.00)
A, Plain: 3.50—4.00 (same as orchestral; possible typo?)
E: 3.20—3.50 (core 2.30—2.50) (compare to orchestral A—probably wound heavier due to lower pitch and a need to maintain a bit more tension)
As you'll see looking in there, the recommended gauge for a G string (especially if tuning up to a higher A) is actually on the light side for what you often see nowadays—that 2.00mm is the lightest gauge for one of Dan's G strings at Gamut, or the ones that Louis just linked. Aquila doesn't offer any lighter than 2.00mm at the 180cm length, although if you talk to Mimmo it might be possible to get lighter gauges than that with enough length to be useful.
Dan's Heavy+ solo a-1 is 1.88mm, so basically equivalent to the 1.90mm. The rest of the options are only lighter from there, so if you look at Gamut, you should be able to fairly easily find something that will match with your Sonores tension-wise.
Also, if you're adventurous, you could even see about tuning a plain gut G/A top string even further to B-flat or B natural—there's documentation of Bottesini playing around with tunings up a third rather than just a whole step. Plain gut can be very forgiving when it comes to that sort of thing.
