[Chalkarts]

I know you can’t harvest reagents but can you buy them?
If so,
Can they be used on alchemical preparations particularly those created in downtime?

[fseperent]

If you can find a talismonger that has reagents aspected to your tradition.

If you use reagents for preparations, the number you used becomes the limit.

As for making preparations during downtime, that would depend on the GM.

[Sphinx]

I know you can’t harvest reagents but can you buy them?
If so, can they be used on alchemical preparations particularly those created in downtime?

In normal games, you can harvest your own reagents (see "Harvesting Reagents," SR5 p.317) as well as buy them (see "Magical Equipment," SR5 p.461).

In Shadowrun Missions, however, you can't collect reagents or craft your own foci (see SRM Combined FAQ v1.1, p.24), but you can still buy magical equipment, and alchemy and preparations are allowed (p.32).

[Michael Chandra]

Forbidden Arcana has new rules regarding costs and availability of Reagents on p187. Since you're not going for Rare Reagents but most likely just regular, you should be able to go for the default price of tainted Reagents. The table is kinder than the initial errata'd one from Street Grimoire, which had Refined and Radical Reagents quickly go up in price.