I have something that I copied from some Facebook page where people were talking about teaching Montessori math. I hate to just copy her advice without giving credit, but it's really good...
For beads - Montessori Outlet is my favorite discount provider. Be sure to buy their 7mm beads, which are all the same size (and color) across material sets. Important because you'll be doing mixing and matching!
- Short bead chains & squares (incl 10s)
- Thousand chain (and hundred if not included in set above)
- "Introduction to Decimal System" set (1-th cube, 9-h sq, 9-t bar, 9 unit beads)
- 100 golden unit beads
- 45 golden ten bars (or use from Decanomial box)
- as many extra WOODEN hundred squares as you can afford - 45 extra wooden ones are nice, but even 10 more is helpful
- (lower priority) as many extra WOODEN thousand cubes as you can afford - pricy, but helpful for multiplication
- Bead decanomial (box of 55 each 1/2/3/4/5/6/7/8/9/10 bars)
- Elementary negative snake game
- Large bead frame (late Primary / early Elem)
Visit Montessori Print Shop for Large Number Cards (Free!), Bead Chain Labels (free for short chains, ~$2 for thousand chain), and Small Number Cards ($2-3?). They also have the Stamp Game, with instructions (awesome!).
Make your own Seguin (teen/ten) boards with poster board - there are online tutorials with measurements.
You'll be missing the long bead chains and cubes, but I think the concept of cubing can be made with just the thousand chain and cube. Plus then you don't have to buy the cabinet for storage (saves you $300-500+)
You'll also need to get creative with trays and what to set out together and when - e.g., just use the first three boxes of the Elementary Snake Game for the Snake Game in Primary. Also, every time you need colored bead bars, just use them from the Decanomial box (Seguin Boards, extra for Snake Game, Multiples, etc.). Then obviously make sure they're all there when you're ready for the Decanomial! Wouldn't hurt to buy a couple extra sets of colored bead bars ($1.95 each).
When you hit early elementary, buy or make a checkerboard (just felt squares glued on corkboard is fine), and the large bead frame (no need for small).
Also, if you're open to alternatives, a few people have recommended RightStart to me. It's developed by a Montessori teacher and uses an abacus.