I'm buying performace springs to lower my car and the company I'm buying from doesn't have the ti listed in their computers and have asked me if 318i springs would fit on the ti. Do they? [just to make sure: springs not suspension].

no

I believe the rear are going to be different.
What company doesn't make ti springs?

Eibach North America (Pro-Kits) are no longer sold for the Ti.

is it shocks and springs or coilovers ?
go to tcklineracing.com and check them out
grab the number and give them a call talk to Pete and he'll be able to help you out ...

Shock and Springs IMO. I seem they have less to break and are more durable. but I am old school in my thinking.

Just buying lowering springs and not complementing with shocks is a very bad idea.

Would 318i springs fit in the front and e30 springs fit in the back?

No e30 will not work...Know from the opposite..I have FK Springs on my 97 when it came over from Germany. Someone tried to put them on a e30 and it didn't work.

Some of what I know is still around on the market for the 318ti.

Eibach Sportlines(Common Brand)
Vogtland(Purple, many ebay stores carry them)
BavAuto
OE Mtechnics (OE Dealer, PelicanParts Special Order)
H&R, various drops (Common Brand)
FK (More of a Euro Brand)
Hartge (out of production, but you are looking for H3 2.1 springs)(Better luck in Germany/Euro market)
AC Schnitzer (Top Money for Springs. Way Over Priced)(BavAuto sells them, IIRC they are white)
Racing Dynamics( Out of Production, but Stock can still be found)

ok i did some playing around and the short answer is *yes* they will fit. here is the catch, the rear springs have a few more dead coils on them so the drop is not as low. for example i bought 96+ M3 H&R Race springs (so i can use the fronts) and a set of used 318ti H&R Race springs (so i could use the rears). for sh*ts and giggles i installed the entire set of M3 race springs on my car. b/c of my swap the fronts were no problem (had to do some hardware changes though). the rears fit no problem as well except the drop was more along the lines of something in between an OE Sport drop and a Sport spring drop. mind you these M3 springs would lower a coupe about 1.5+ inches b/c of different geometry. the handling was superb but the drop was off. so in short they will fit but the advertised drop will not be what you get.

my friend (Phamsta) actually installed the rear M3 springs on his Ti b/c he liked the subtle drop with the increased stiffness/handling.