Mother Shahraz is one of the few fights in Burning Crusade that requires resistance. She does a huge amount of raid damage, all shadow. She also does a wonky teleport + AoE thing that's resistable, a very good thing.
First off, if you're there as a tank you don't need any resistance gear. Instead you're mostly worried about the melee damage she does (Saber Lash, if you're an off-tank). Just wear your best survival suit and you'll be fine.
If you're DPS, the common wisdom is you want at least 300 shadow resistance. You'll get 70 from Shadow Protection, so you want at least 230 from gear. Fortunately that's pretty easy, between the BT necklace and the BT crafted gear. Here's what I wear:
40 Medallion of Karabor, quest reward from the BT attunement
40 Night's End, crafted (tailoring)
40 Redeemed Soul Wristguards, crafted (leatherworking)
54 Redeemed Soul Cinch, crafted (leatherworking)
54 Redeemed Soul Moccasins, crafted (leatherworking)
20 Glyph of Shadow Warding, Lower City Honored
None of the above is too hard for a BT guild to get. The Medallion takes some doing, but if you run SSC and TK regularly you can get it done on the way. The craftables are all things any crafter running BT regularly can make. The real hassle is getting the Hearts of Darkness, hopefully your guild has been saving those away all along while working through the content. Basically the SR requirement is just a check Blizzard put in to make sure that raiders didn't take too many shortcuts before getting there.
The Glyph is an extra bit of sauce; I tossed it on an old T4 helm I had since 2/5 T4 is so useful for cat DPS. Another option is +15 shadow resist to back, but it takes a lot of Primal Shadow and so is expensive. You can also get the old +10 shadow resist to back enchant, that's pretty cheap. Finally you can add +8 resist to chest, legs, hands, or feet with a Shadow Armor Kit, but honestly I wouldn't bother.
That gear gets me to 248. There's also some Redeemed Soul Legguards for another 72, but they're expensive and most people feel it's overkill. OTOH, with the way resistance works the higher you go, the more return you get per point of resist gear. 300 resistance is about 60% resistance; 365, the resist cap, is 75%.
The shadow resist gear is total trash in every other respect. Nothing but stamina and SR. Even so I was able to pull about 1000 DPS on Mother. Good enough, particularly since the fight isn't a big DPS race.
I'm really glad Blizzard mostly moved away from resist sets as a game design element.