Hi Craig, I may have what you are looking for. I have a Standish "Attack" 24" blue boys MTB with suspension forks and running shimano SIS gears (at least thats what it says on the rear derailleur).
I bought it a couple of years ago for my youngest daughter as a step up bike (when she outgrew her 20" bike) but ended up buying both my girls new mountain bikes.
If you are interested I can send a photo or two. Price $30. At the moment it is just gathering dust in the shed.
My son has outgrown his 24" GT Aggressor. We bought it secondhand about 2 1/2 years ago and it's been a great little bike - we've taken it on several bike holidays, done Amy's Ride etc. To look at, I'd say it's in good rather than excellent condition. The frame/forks have some scratch and rub marks on the paint and decals - mostly, I suspect, from being on a bike rack. However, drivetrain is great - the chain and cassette are reasonably new, the gears change nicely, the brakes are good and the wheels/tyres are fine. I'm pretty handy with bikes so it's been well maintained.
Anyway, this is the model: http://www.comparestoreprices.co.uk/cycling/gt-aggressor-24-inch-20... (it looks basically the same as this - the only difference I can see is that mine has SRAM Gripshifts not Shimano gears - they work very nicely).
I reckon I'd want $75 for it. I'm happy to send some pics if you are interested.