You might want to find out if Geo Sans Light from Manfred Klein has licensing compatible with your needs. Based on his "simple terms of use," it seems to me that you have to ask for permission, but it is very likely he will grant it considering that he makes his fonts free for charity use.

It doesn't have the 'a' you need but how about Bitstream's Vera?

Apart from that Raph's suggestion sounds good. You could check out the so called "free fonts" on the Web, find something that works for you then track down the designer and ask for formal written permission to redist. Tricky part will be finding a font that isn't a renamed or in some other way derived from a commercial font. The Typophiles here should be able to help you identify a low-risk font from a list of alternates and named designers.

Finally have you tried talking to Monotype and other commerical foundries with fonts along the lines of what you need? You never know they may be able to cut you a deal.

Cheers, Si