3D printed plastic bases is about the most difficult way to get them. It's almost like asking someone to carve them from a plastic block ;)
Assuming you're a USAian, people used to swear by Chessex's plastic hex bases. But I can't find any on their website. Anyone else got details?
Also there used to be a company called Proxy Models, they appear to be on Amazon. BT hexes are 1.25", which is 30mm, point to point - not flat to flat.
Reaper have 1" plastic bases. From my experience with them from the CAV minis, they're slightly larger than a BT hex but entirely usable.
https://www.reapermini.com/miniatures/hex/basesAs a non-USAian, I enjoy not paying enormous postage fees, and get my hex bases from Ral Partha Europe.
http://www.ralparthaeurope.co.uk/shop/battletech-c-1/miniatures-c-1_3/accessories-c-1_3_68/99125-plastic-hex-bases-10-p-1858.html I usually get bulk lots of 10 packs at a time. RPE also has metal hex bases, as does IWM of course.
Some folk worry about models overtoppling on plastic bases. I've found that unless you're dealing with very edge cases, as long as you keep the centre of gravity of the mini over the base, you're right. so mount a front-heavy mini at the back of the base, and so on.
Best place for (technical term) flocking is your local hobby store, or any railway hobby store. Get a couple of different types - burnt grass, summer grass, run wild. The "bushes" material is also useful.
Best place for sand & dirt is your local garden supplies store. Many have mounds of sand, gravel, dirt, you name it, in a variety of colours. Tell them you'd like some samples for a project you're planning - they'll give you plastic bags, and you can score a pound or so of each for free. That'll last most of your lifetime :) When you get them home, I recommend running them through a sieve to remove stick, insects, bird crap etc; put them on a baking tray somewhere inside to dry for a couple of days. Then jar up, and you're good!
I tend to get "little gravel" bits from the roadside, local building sites, etc.
And off the wall suggestions:
- the best Martian surface material is the "dust" from around clay tennis courts. If you've been to one, you know exactly what I mean!
- Coffee grounds make a great dark earth. Just they're much lighter than dirt/sand, so you need to press them down into the glue.
Some people user superglue for the glueing; I prefer white glue (like from school) as it's more forgiving, and less likely to weld my fingers to the base.