There are several good sources for aftermarket blocks, deadeyes, and other fittings. Lots of modelers like the wood ones available through Model Expo, but my personal favorites are the cast britannia metal ones from Bluejacket ( www.bluejacketinc.com ). They have to be painted (or "blackened" with a chemical called "Pewter Black," which Bluejacket also sells), but they're far better detailed and more accurately proportioned than any of the European ones I've gotten from Model Expo.
The bad news is that Bluejacket raised the prices of its fittings a couple of weeks ago. The good news is that a Santa Maria doesn't need many of them.
It's awfully difficult to suggest what types, sizes, and numbers of fittings you need. Part of the problem is that we know so little about ships of the fifteenth century. Some authorities think the Santa Maria's shrouds were set up with deadeyes; others think simple block-and-tackle arrangements would be more appropriate for the period. (I'd vote for the deadeyes; they show up in quite a few contemporary paintings and drawings.)
The second question is how much you want to modify the kit. I haven't looked at the Heller Santa Maria for many years, but I imagine it contains some representations of blocks and deadeyes. Many plastic sailing ship kits represent deadeyes by molding them integrally with the lanyards (the ropes that connect the pairs of deadeyes together). One of the most effective ways to improve a large-scale model like this is to replace those moldings with "real" deadeyes and lanyards. You can count the ones in the kit and order Bluejacket "Old Style" deadeyes - half "upper" and half "lower." I suspect at least two sizes would be appropriate - one for the fore and main lower deadeyes, one for the mizzen and main topmast deadeyes. The two smallest sizes Bluejacket offers would be about right.
To figure out how many blocks you need, and of what sizes, you really have to have a rigging diagram. My guess is that whatever came with the Heller kit is useless. (Those Heller designers were wonderful artisans, but experience makes it pretty clear that they had no understanding of how a ship's rigging works.) My suggestion is to sketch out a rigging diagram yourself, using any of those books that I recommended earlier as your starting point. You'll quickly figure out, I think which lines are particularly important and which ones can reasonably be omitted. You'll probably want to include all, or nearly all, of the standing rigging (the lines that support the masts). Some of the more intricate running rigging (the lines that control moving components, including yards and sails) can be omitted on a model. When you've got your diagram in hand you can make an inventory of the blocks you'll need, and send in your order.
You may want to order a small stockpile of blocks in advance. I suspect if you order the following you won't be wasting your money:
2 dozen single rope-stropped blocks, 3/32"
2 dozen single rope-stropped blocks, 1'8"
1 dozen double rope-stropped blocks, 3/32"
1 dozen double rope-stropped blocks, 1/8"
That probably won't be enough to rig the whole model, but it'll get you started.
If you do use Bluejacket blocks, you'll need a couple of tools: a pin vise with a set of #60 to #80 drill bits (for reaming out the holes) and a small file with at least one sharp edge (for cleaning out and fully defining the grooves). Be warned: the flash and general crud on those 3/32" single blocks takes some time to clean off - and they're mighty small. You'll probably get pretty frustrated filing out the grooves on the first few. Your fingers, however, will follow the usual learning curve (ship modeling has lots of short but steep learning curves). My little model of the frigate Hancock required several hundred of those 3/32" single blocks, and I survived. And the result, in my opinion, was worth the effort.
This is a good project for getting acquainted with ship model rigging. Good luck.