Now, it happens that if you're collecting Marines or Orks, you can usually get massive lots of stuff for very cheap off eBay, since those two armies were in every starter box ever and everybody has a whole lot. Eldar is tricky, since it's a very unique aesthetic, but for Guard especially (conveniently for job) there are a lot of manufacturers making strong sci-fi lines in 6mm.
Dark Realm is a good place to start (and quite a few online retailers carry them). Their Pax Arcadia line is a rather orthodox line of tanks and infantry, perfect for Guard. Andrayada have some nice high-tecchy infantry that could pass for either Eldar or Necrons. Their vehicles are all kind of chunky walkers, some of which make good Space Marine Dreadnoughts. The Kraytonian line has a bunch of skimmer vehicles that could pass for Eldar, but most of their infantry are kind of lizardy. Finally, Dark Realm's Skyth are bio-beasts that I'm going to be using to supplement my Tyranids. (Price note: 4 Rhino-analogues are 6.50GBP, compared to 15.50GBP for 6 Rhinos from GW.)
Steel Crown's Exodus Wars line also has some nice subs. I'm using their Edenites for my Necron infantry, and they have Squat-analogs (Khazari) with some really excellent aircraft to use as Valkyries / Vultures. The Guild, like Pax Arcadia, are straight-up humans that work well for Guard, and it looks like their new Royal Empire line is shaping up the same way. (Price note: 4 Valkyrie-analogues - helpfully called Valkyries
- are 4.95GPB, compared to 12.30GBP for 2 Valkyries from GW.)Finally, Brigade has a nice range of 6mm vehicles. Most of them are high-tech human-looking hovercraft of various stripes, but their Neo-Soviet line has some great tracked vehicles - my own Manticores are Vikhr Katyushas from that range. (Price note: The Neo-Soviet Basilisk-analogues are 1GBP each, compared to an artillery company of 10 vehicles for 30.75GBP from GW.)
Also, of course, for Guard and their WWI / WWII aesthetic, historical manufacturers work well. If I was going to do a siege regiment, I'd use models from GHQ or CinC, both of whom have extremely comprehensive historical ranges.
Ground Zero Games is also probably worth mentioning for a strong SF 6mm line, although they have a kind of poo webstore with very few pictures.
(Edit: Now, of course, I am kicking myself for not picking up a blister of not-Vultures from Exodus Wars for my Guard force when I placed my not-Necron order.
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