I don't use the sleeves, I find they deform the seats too much and aren't needed to locate the seat.
My daughter always goes on the nearside, but I'm not sure there's a recommended side as such. Statistically that side would keep her away from oncoming traffic more, and of course it's also kerbside.
Local TPS or Audi for the bars and bolts. Nowhere else sells them new. Try eBay for second hand but a lot of them are up the £70 mark.
Part numbers -
8E0813545A - N/S isofix bracket
8E0813546A - O/S isofix bracket
8L0837453 bracket bolts - 2 per bracket.