I'm pretty sure you'd have to make them to order; though thanks to the way cutters work, you could score the bases to put them on as well.
From experience, you can get about 77 25mm bases from an A4 sheet of plywood or perspex, so adding 2mm rings to that would probably give you in the region of 60.
A decent laser cutting service will charge you maybe $10 for the job + materials. 3mm plywood i've found to be pretty cheap at about $2/ A4 sheet.
Attached is an example using a 1.5mm perspex sheet for 25mm bases with precut holes for pins in the guardsman's feet against a regular textured base.
Alternatively, you could adjust the bases to be, say 23mm and have 23.1-25mm rings. This way you could keep the base sizes consistent and be able to see squad setups from on top as well as from the side. Speaking as someone who tried to do the painting base side approach once (and only once), this will save you a HUGE amount of time, and gives your army a nice
RTS interface feel with selection rings around the models
If you want to go this road, i'm happy to share my design documents as i'm rebasing my entire collection with transparent bases.