"if I buy a pre-paid SIM card in Croatia, then I will have access to mobile and text messaging, correct?"
Yes. You would be subject to the terms of the plan you bought. For instance, you may have to pay per text, or there may be a certain number of texts included.
"Would I need to buy another one to go into Slovenia (or would they perhaps have a parent company allowing service across borders)."
When you go into Slovenia with a Croatian SIM, the phone will still work, but you are roaming. You would then be paying at whatever terms your plan had for roaming. Within the EU, roaming rates are capped; since both of these countries are part of the EU, your roaming rates should not be high. But be careful if you are going outside the EU (say, to Montenegro); these rates are not regulated, so they can be low or high. A friend of mine took his T-Mobile UK SIM to Turkey. It has very good rates within the UK, from the UK to the US, and while roaming in the EU. But on my advice, he checked on roaming rates in Turkey, and they were £1.50 per minute - ouch!
"Still would have no data, unless I buy a data plan somewhere, correct?"
Again, that would depend on the terms of the plan you bought when you bought your SIM. These days, most plans include data, but you have to double-check the exact terms. If you are going to use a lot of data, you want a plan that's advantageous for that (for instance, one that includes a gigabyte or two per month, rather than one that charges by the megabyte).
"Where is the best place to buy a pre-paid SIM card?"
They're sold all over the place - newsstands, kiosks, and dedicated stores (like AT&T stores in the US). Once you know which company you want, ask at your hotel for the nearest place that sells it.
To learn more about the subject, a great source of info is Prepaid GSM. Here's their Croatia page (next to "Prepaid Offer" select "click here" to learn more about each company's options): http://www.prepaidgsm.net/en/croatia.php
And here's their Europe Forum. Be aware that information there can get VERY technical; don't be afraid to ask for clarification: http://www.prepaidgsm.net/forum/forumdisplay.php?f=22