The 3.0 Monster Manual has illithid stats for 3.0 D&D. The half-illithid template, along with a sample half-illithid lizardfolk, is in the Fiend Folio. The Fiend Folio is sort of a transitional book between 3.0 and 3.5 I think, so just be aware that it has some changes to creature types, subtypes, and the basic stats for some creature types. The illithilich, or alhoon, is described in brief at the back of the Monstrous Compendium: Monsters of Faerun, for 3.0. I think the standard illithid is also duplicated with a racial level progression in Savage Species, for 3.0, but like the Fiend Folio that book is sorta inbetween 3.0 and 3.5 in its rules.
You won't find the illithid stats online, as illithids are one of the critters designated as Product Identity or somesuch and, thus, illegal to reproduce in print or online except by Wizards of the Coast. Don't ask us to post the illithid stats or description.
If you want to run a 3rd Edition campaign of any sort, it's almost essential to buy the three core rulebooks, though you could manage with just the online copies of the free SRD (System Reference Document). The SRD contains most Open Game Content from 3rd Edition D&D. But experience point charts, basic character level advancement stuff, suggested average wealth by level charts, and such are only available in the actual Player's Handbook and Dungeon Master's Guide. If you don't want to spend more money on getting the 3E DMG and MM for running your games, then you'll just have to come up with your own versions of the aforementioned XP/wealth/advancement charts, and make up your own version of 3E illithid stats based on what precedents you see in the books you have and in the SRD.
There's a PDF product available from Lion's Den Press, if I recall correctly, called the Iconic Bestiary, which has Open Game Content versions of various iconic D&D monsters that cannot be reproduced; the iconic monsters in that product are just vaguely similar to the D&D monsters they mimic, filling more or less the same roles as such critters in D&D play. They have different names and stats, but generally resemble the un-reproducible D&D monsters such as illithids and yuan-ti. Buying this PDF online should be cheaper than buying the Monster Manual itself.
*hopes he didn't get mixed up on the name of the PDF's producer*
I don't think they did changed a thing from the old edition. They did add a LA to it, so that they're marginally playable as PC classes, but you need to start of as level 15 or 16 characters to pull that off, I believe...
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