This is a really great topic- thanks for bringing it up, @AmethystGlynae! You’ve got some awesome advice already and Francisco shared a great book for finding corresponding/similar ingredients 
When I’m short of an ingredient (especially if it’s a crystal or incense), I usually just leave it out entirely and work with what I have on hand. I’m more familiar with herbs and plants, so if I’m missing an herb for spellwork or cooking, I usually take a look at it’s purpose and then find something similar 
While lavender is one of the main soothing herbs, In Herbs for Anxiety I also explore:
- Chamomile- Limeberry mentioned this and I totally agree!
- Lemon Balm (sometimes called Melissa)
Lavender is used for healing as well, especially in skin conditions like burns and scarring. For these purposes, I would suggest switching it for:
- Garlic
- Calendula
- Honey
- Aloe Vera
If it is the purple color important to the spell or presentation, consider:
- Heather
- Cone Flower/ Echinacea
- Butterfly Sweet Pea
Hopefully substituting with another herb can replace lavender while not aggravating your allergies. If the spell you are doing is being consumed (like tea) definitely check with an allergist or doctor before trying new herbs! Especially if you are prone to serious reactions.
Good luck- I hope you can find the perfect substitution! Blessed be 